I've always enjoyed figuring out how to solve brain teasers
and I've heard that your
brain is like a muscle so if you keep it in shape (by teasing it I guess)
it will serve you well
throughout your life. Even if that's not true, I still like to solve them.
If you know of any more good ones please send them to me. It can also be
useful to know some of these if you are interviewing for a job somewhere.
Lots of interviewers like to
use these questions to ascertain how quickly you can think on your feet. If
you don't understand the answer, you can email me and I'll explain it to you. Have fun :)
By moving one of the following digits, make the sum correct. 62 - 63 = 1
26 - 63 = 1 (In other words, 2x2x2x2x2x2, which equals 64)
You have a fox, a chicken and a sack of grain. You must cross a
river with only one of them at a time. If you leave the fox with the
chicken he will eat it; if you leave the chicken with the grain he will
eat it. How can you get all three across safely?
Take the chicken over first. Go back and bring the grain next, but
instead of leaving the chicken with the grain, come back with the chicken.
Leave the chicken on the first side and take the fox with you. Leave it
on the other side with the grain. Finally, go back over and get the
chicken and bring it over.
You have 12 black socks and 12 white socks mixed up in a drawer.
It's early in the morning and you don't have any light to see the colors.
How many socks must you pull out (blindly) to be sure of getting a
matching pair?
3 socks. If the first sock is black, the second one could be black,
in which case you have a matching pair. If the second sock is white,
the third sock will be either black and match the first sock, or white
and match the second sock.
What is special about the following sequence of numbers?
8 5 4 9 1 7 6 10 3 2 0
The numbers are in alphabetical order.
(eight, five, four, nine, one, seven, six, ten, three, two, zero)
Three travelers register at a hotel and are told that their rooms
will cost $10 each so they pay $30. Later the clerk realizes that
he made a mistake and should have only charged them $25. He gives
a bellboy $5 to return to them but the bellboy is dishonest and
gives them each only $1, keeping $2 for himself. So the men actually
spent $27 and the bellboy kept $2 - what happened to the other
dollar of the original $30?
Nothing. The 3 travelers paid a total of $27, making $25 for the hotel
and $2 for the clerk. There is no missing $1.
You are the bus driver. At your first stop, you pick up 29
people. On your second stop, 18 of those 29 people get off, and at the
same time 10 new passengers arrive. At your next stop, 3 of those 10
passengers get off, and 13 new passengers come on. On your fourth stop
4 of the remaining 10 passengers get off, 6 of those new 13 passengers
get off as well, then 17 new passengers get on. What is the color of
the bus driver's eyes?
U2 has a concert that starts in 17 minutes and they must
all cross a bridge to get there. All four men begin on the
same side of the bridge. You must help them across to the
other side. It is night. There is one flashlight. A
maximum of two people can cross at one time. Any party who
crosses, either 1 or 2 people, must have the flashlight with
them.
The flashlight must be walked back and forth. It cannot be
thrown and other tricks like that are not needed to solve the problem.
The solution is simply a matter of allocating resources in a certain
order. Each band member walks at a different speed.
A pair must walk together at the rate of the slower man's pace:
Bono: 1 minute to cross
Edge: 2 minutes to cross
Adam: 5 minutes to cross
Larry: 10 minutes to cross
For example: if Bono and Larry walk across first, 10 minutes
have elapsed when they get to the other side of the bridge.
If Larry then returns with the flashlight, a total of 20
minutes have passed and you have failed the mission.
You get to figure this one out on your own :) If
you can't figure it out, email me
and I'll give you a hint. There is an answer, and it doesn't require
tricks like throwing the flashlight or shining it backwards. You just
have to find the combination of people crossing back and forth in the
normal way (there are two of them) that takes 17 minutes.
Why is it very common to have a 9 minute snooze interval on alarm
clocks, why not 10 instead?
By setting the snooze time to 9 minutes, the alarm clock only needs to
watch the last digit of the time. So, if you hit snooze at 6.45, the
alarm goes off again when the last digit equals 4. They couldn't make it
10 minutes, otherwise the alarm would go off right away, or it would take
more circuitry.
A bookworm eats from the first page of an encyclopedia to the last
page. The bookworm eats in a straight line. The encyclopedia consists
of ten 1000-page volumes and is sitting on a bookshelf in the usual
order. Not counting covers, title pages, etc., how many pages does the
bookworm eat through?
so the bookworm eats only through the cover of the first volume, then
8 times 1000 pages of Volumes 2 - 9, then through the cover to the 1st
page of Vol 10. He eats 8,000 pages.
An Arab sheikh tells his two sons to race their camels to a distant
city to see who will inherit his fortune. The one whose camel is
slower will win. The brothers, after wandering aimlessly for days, ask
a wise man for advise. After hearing the advice they jump on the
camels and race as fast as they can to the city. What does the wise
man say?
An 18-wheeler is crossing a 4 kilometer bridge that can only
support 10,000 kilograms and that's exactly how much the rig weighs.
Halfway across the bridge a 30 gram sparrow lands on the cab, but the
bridge doesn't collapse. Why not?
Since the bridge is 4 kilometers
long, the halfway point would be 2 kilometers. The 18-wheeler would
have used much more than 30g of fuel to drive 2 kilometers.
A completely black dog was strolling down Main Street during a
total blackout affecting the entire town. Not a single streetlight had
been on for hours. Just as the dog was crossing the middle line a Buick
Skylark with 2 broken headlights speedily approaches his position, but
manages to swerve out of the way just in time. How could the driver have
possibly seen the dog to swerve in time?
Ida puts her coffee into the microwave, as she does every morning,
for exactly 2 minutes. When the microwave goes off, she opens the door,
but then closes the door again and sets the microwave for 2 more seconds.
What good would 2 more seconds be?
You are a cook in a remote area with no clocks or other way of keeping time other than a four minute sandglass timer and a seven minute sandglass timer. (The kind you turn over - hourglass shaped) You do have a stove, however, with water in a pot already boiling. Somebody asks you for a nine-minute egg, and you know this person is a perfectionist and will be able to tell if you undercook or overcook the eggs by even a few seconds. What is the least amount of time it will take to prepare the egg? And how will you prepare it so that it is neither undercooked or overcooked?
The answer is 9 minutes. First, flip both hourglasses over and drop the egg into the water. When the four minute timer runs out, flip it again. When the seven minute timer runs out, flip it over. The egg has been cooking seven minutes. Now when the four minute timer runs out again (after eight minutes) flip the seven minute timer back over. Since the seven minute timer has been running only a minute between flips, there's a minute worth of sand left. And when that minute runs out, the egg will have been cooking for exactly nine minutes.
I am the owner of a pet store. If I put in one canary per cage, I have one bird too many. If I put in two canaries per cage, I have one cage too many. How many cages and canaries do I have?
If you put one canary in each cage, you have an extra bird without a
cage. However, if you put two canaries in each cage then you have two
canaries in the first cage, two canaries in the second cage and an
extra cage.
Here is a series of numbers. What is the next number in the sequence? 1 11 21 1211 111221 312211 13112221
The next number in the sequence is 1113213211, because the rule for creating the next number is to simply describe the previous number. For example, you start of with 1, with is simply one 1, so the next number is 11. Now you have two 1's, so the next number is 21. Now you have one 2 and one 1, so the next number is 1211. The solution is to simply continue describing the previous number using only numbers.
My daughter has many sisters. She has as many sisters as she has
brothers. Each of her brothers has twice as many sisters as brothers.
How many sons and daughters do I have?
Two women apply for a job. They are identical. They have the same
mother, father and birthday. The interviewer asks, "Are you twins?"
to which they honestly reply, "No".
You are standing outside a closed door. On the other side of the
door is a room that has three light bulbs in it. The room is completely
sealed off from the outside. It has no windows and nothing can get in or
out except through the door. On the outside of the room there are three
light switches that control each of the respective light bulbs on the
other side of the door.
Your assignment is to determine which light switch controls which light
bulb. You are allowed to enter the room only once, and once you come
out, you must be able to state with 100% certainty which light switch
controls which light bulb.
Turn one light switch on, wait a few minutes, then turn it off and turn
another light switch on. Go into the room and feel the light bulbs.
The one that's still warm is connected to the switch that you first
turned on, the one that is on was the second switch you turned on, and
the last bulb is controlled by the switch that you didn't touch.
If a bottle and a cork cost a dollar and a nickel, and the bottle
costs a dollar more than the cork, how much does the cork cost?
Most people guess 5¢, but $1 more than 5¢ is $1.05, and if the
bottle cost $1.05, the bottle and the cork would be $1.10, not $1.05.
The cork actually costs 2½¢ and the bottle costs a
dollar more, or $1 and 2½¢, making the total $1.05.
A boat has a ladder that has six rungs. Each rung is one foot apart.
The bottom rung is one foot from the water. The tide rises at 12 inches
every 15 minutes. High tide peaks in one hour.
When the tide is at its highest, how many rungs are under water?
None. The boat is floating on the water, so as the tide rises, so does
the ladder.
You have a lighter and two fuses that take exactly one hour to
burn, but they don't burn at a steady rate. For example, one fuse
could take 59 minutes to burn the first inch and then burn the rest
of the fuse in the last minute.
How would you use these two fuses to measure 45 minutes?
Light the first fuse on both ends and the second fuse at only one end.
When the first fuse burns out you know 30 minutes have passed. Light the
other end of the second fuse and when it burns out, 45 minutes have passed.
You have two buckets - one holds exactly 5 gallons and the other
3 gallons. How can you measure 4 gallons of water into the 5 gallon
bucket?
(Assume you have an unlimited supply of water and that there are no
measurement markings of any kind on the buckets.)
Pour the 3 gallons of water into the 5-gallon bucket
Fill the 3-gallon bucket again.
Fill up the 5-gallon bucket with the 3-gallon bucket, leaving you with 1 gallon left in the 3-gallon bucket.
Empty out the 5-gallon bucket.
Pour the remaining 1 gallon of water from the 3-gallon bucket into the 5-gallon bucket.
Fill the 3-gallon bucket.
Pour the 3 gallons of water from the 3-gallon bucket into the 5-gallon
bucket leaving you with 4 gallons of water in the 5-gallon bucket.
A princess is as old as the prince will be when the princess is
twice the age that the prince was when the princess's age was half the
sum of their present ages.
This one took a while, but I figured it out. You can
find the answer here.
During WWII, there was a bridge connecting Germany and Switzerland,
and on the German side, there was a sentry tower with a guard in it.
He would come out every three minutes to check on the bridge, and he had
orders to turn back anyone who tried to get into Germany, and shoot
anyone trying to escape without a pass. There was a woman who
desperately needed to get into Switzerland, and she knew she didn't
have time to get a pass. It would take her at least six minutes to
cross the bridge, but she managed to do it. How?
She walked on the bridge towards Switzerland for 3 minutes and just
as the guard was about to come out, she turned around walking back to
Germany. The guard saw her and asked for her pass but she didn't have one
and was sent back (or what the guard thought was back) to Switzerland.
In her case it was the very country she wanted to go to.
A man can make perfect counterfeit bills. They look exactly like
real ones, they're made of exactly the same materials, made the same way,
everything. So perfect, one could pretty much call them real bills.
One day he successfully makes a perfect copy of another bill. However,
he gets caught when he tries to use the copy. How is this possible?
The trick is that the word dead represents a number in
hexadecimal. That number in base 10, plus one to include
yourself, is: 57005 + 1 = 57006.
A man is traveling with a fox and two chickens, if he leaves the
fox alone with the chickens the fox will eat the chickens. He comes to a
river and needs to cross it, he finds a small boat that can carry only
him and one animal, how does he get himself, the fox and two chickens
across the river safely?
Take the fox over, return with nothing. Go over with one chicken,
return with the fox. Go over with the second chicken, return with
nothing. Finally, take the fox over.
A man is looking at a picture of a man on the wall and states:
Brothers and sisters I have none, but this man's father is my father's
son. Who is the man in the picture in relation to the man looking at
the picture?
The man in the picture is his son. Since he doesn't have any brothers
or sisters, the statement my father's son is himself.
A shortened version would be this man's father is myself, so he is the
father of the man in the picture.
A man and his son had a terrible car accident and were rushed
to the hospital. The man died on the way, but the son was still alive
and a surgeon was called in to operate. However, the surgeon saw
the young boy and said, "I can't operate on this boy. He's my son."
How is this possible?
A wise king devised a contest to see who would receive the
Princess' hand in marriage. The Princess was put in a 50x50 foot
carpeted room. Each of her four suitors were put in one corner of the
room with a small box to stand on. The first one to touch the Princess
hand would be the winner and become the new King.
The rules of the test were that the contestants could not walk over the
carpet, cross the plane of the carpet, or hang from anything; nor could
they use anything but their body and wits (i.e. no magic or
telepathy, nor any items such as ladders, block and tackles etc).
One suitor figured out a way and married the Princess and became the
new King. What did he do?
Two guards were on duty outside a barracks. One faced up the road
to watch for anyone approaching from the North. The other looked down the
road to see if anyone approached from the South. Suddenly one of them
said to the other, "Why are you smiling?"
They were facing each other. As to why his companion was smiling, the
world may never know.
You're riding a horse. To the right of you is a cliff and in
front of you is an elephant going the same pace as you and you can't
overtake it. To the left of you is a hippo running at the same speed
and behind you is a lion chasing you. How do you get to safety?
You have 50 quarters on the table in front of you. You are
blindfolded and cannot discern whether a coin is heads up or tails up by
feeling it. You are told that x coins are heads up, where 0 < x < 50.
You are asked to separate the coins into two piles in such a way that the
number of heads up coins in both piles is the same at the end.
You may flip any coin over as many times as you want. How will you do it?
Take x coins, flip all of them and put them in one pile. The rest of the
coins form the second pile.
You have four chains. Each chain has three links in it. Although
it is difficult to cut the links, you wish to make a single loop
with all 12 links. What is the fewest number of cuts you must make to
accomplish this task?
3 cuts. Cut each link in one chain. Separate them, and use the links
to join the ends of the 3 intact chains.
Walking down the street one day, I met a woman strolling with her
daughter. "What a lovely child," I remarked. "In fact, I have two
children," she replied. What is the probability that both of her
children are girls?
1/2 probability. There was a raging debate on this brain teaser,
which I got from 3quarksdaily, and after many comments, the
conclusion was that the problem is stated poorly, but there is a
1/2 probability that both of her children are girls.
Three closed boxes have either white marbles, black marbles or
both, and they are labeled white, black and both. However, you're
told that each of the labels are wrong. You may reach into one of the
boxes and pull out only one marble. Which box should you remove a
marble from to determine the contents of all three boxes?
The one labeled both. Since you know it's labeled incorrectly, it
must have all black marbles or all white marbles. After you determine
what it contains, you can identify the other two boxes by the process
of elimination.
A glass of water with a single ice cube sits on a table.
When the ice has completely melted, will the level of the water have
increased, decreased or remain unchanged?
Remain unchanged. When frozen, the ice cube displaces its weight.
You are given eight coins and told that one of them is counterfeit.
The counterfeit one is slightly heavier than the other seven.
Otherwise, the coins look identical. Using a simple balance scale,
can you determine which coin is counterfeit using the scale only once?
I tricked you. It can't be done with just one try. But it can be done
if you use the scale twice. Weigh three coins with three other coins. If
they're the same, then weigh the two remaining coins and the heavier
one is the counterfeit. If one of the groups of three was heavier,
weigh two of that group of three. If they're even, the third is the
counterfeit. Otherwise, the heavier one is the counterfeit.
You are given eight coins and told that one of them is counterfeit.
The counterfeit one is slightly heavier than the other seven. Otherwise,
the coins look identical. Using a simple balance scale, can you determine
which coin is counterfeit using the scale only twice?
First weigh three coins against three others. If the weights are equal,
weigh the remaining two against each other. The heavier one is the
counterfeit. If one of the groups of three is heavier, weigh two of
those coins against each other. If one is heavier, it's the
counterfeit. If they have equal weight, the third coin is
the counterfeit.
I was visiting a friend one evening and remembered that he had
three daughters. I asked him how old they were. "The product of their
ages is 72," he answered. Quizzically, I asked, "Is there anything else
you can tell me?" "Yes," he replied, "the sum of their ages is equal
to the number of my house." I stepped outside to see what the house
number was. Upon returning inside, I said to my host, "I'm sorry, but I
still can't figure out their ages." He responded apologetically,
"I'm sorry, I forgot to mention that my oldest daughter likes strawberry
shortcake." With this information, I was able to determine all of
their ages. How old is each daughter? You have enough information to
solve the puzzle.
3, 3, and 8. The only groups of 3 factors of 72 to have non-unique sums
are 2, 6, 6 and 3, 3, 8 (both add up to 14). The presence of a single
oldest child eliminates 2,6,6.
You're in a room with two doors. There's a guard at each door. One
door is the exit, but behind the other door is something that will kill
you. You're told that one guard always tells the truth and the other guard
always lies. You don't know which guard is which. You are allowed to ask
one question to either of the guards to determine which door is the exit.
What question should you ask?
Ask either guard what door the other guard would say is the exit, then
choose the opposite door.
If you ask the guard who always tells the truth, he knows the other guard
would lie, so he'll point you to the door leading to death. If you ask
the guard who always lies, he knows the other guard would truthfully
show you the exit, so he'll lie and point you to the door leading to
death.
$1. If eggs are 12¢ a dozen, each egg costs 1¢, so 100 eggs
would be 100¢, or $1.
You are a prisoner sentenced to death. The Emperor offers you a
chance to live by playing a simple game. He gives you 50 black marbles,
50 white marbles and 2 empty bowls. He then says, "Divide these 100
marbles into these 2 bowls. You can divide them any way you like as long
as you use all the marbles. Then I will blindfold you and mix the bowls
around. You can then choose one bowl and remove one marble. If the
marble is white, you live, but if the marble is black...you die.
How do you divide the marbles up so that you have the greatest
probability of choosing a white marble?
Place 1 white marble in one bowl, and place the rest of the marbles in
the other bowl (49 whites, and 50 blacks).
This way you begin with a 50/50 chance of choosing the bowl with just one
white marble and living. But even if you choose the other bowl, you still
have an almost 50% chance of picking one of the 49 white marbles. There
are no guarantees in life, but this is your best bet at surviving.
