11 

Choose the Next Letter 
3 

What am I? 
9 

Mutilated Chessboard Problem 
1 

Greatest Area 
3 

The fake coin 
3 

Surgery 
2 

What Am I? 
7 

Burning Rope 
5 

Psycho Killer 
16 

3 Men and a Hotel Room 
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No if() Only abs()
Great question! Thoroughly enjoyed thinking about this one. context 
Clock Works  Google Interview Question
Great puzzle to ask young kids! context 
Balloon in an Elevator
Yes that's a valid point. I was assuming air drag was not accounted for (i.e. moments after the cable snaps.) context 
Balloon in an Elevator
The fundamental reason why the helium filled balloon floats is buoyancy. Buoyancy is a byproduct of acceleration from the natural state of falling with gravity. In this case, he floor of the elevator is accelerating at exactly +g to stop you from descending. In free fall, the effective acceleration within the elevator is reduced to zero (or very close to it) and buoyancy no longer plays a factor. Thus the balloon stays where it is. This is the same fundamental reason why astronauts on the ISS float. context 
No if() Only abs()
Jeez, I'm going in circles here. ravi's reply is incorrect. That whole expression simply equals 1/2. As long as a and b are not both equal to 0, the following would suffice: {a + b + abs(ab)}/{(a + a + b + b)/(a+b)} context 
No if() Only abs()
Or: {a + b + abs(ab)}/(a/a + a/a). Slightly shorter version. context 
No if() Only abs()
This, however, of course wouldn't work if a was 0. So perhaps the parent's reply is actually more general. context 
5
2
Clever Commuter
Every day, Fred takes the train to travel from his work back to Alkmaar, his place of residence. Usually, he arrives at the station of Alkmaar at six o’clock, and exactly at that moment he is picked
...

Hard

5
12
Crazy Google Interview Question
You are shrunk to the height of a nickel and your mass is proportionally reduced so as to maintain your original density. You are then thrown into an empty glass blender. The blades will start moving
...

Doable

3
3
Time To Fuse
You have a stringlike fuse that burns in exactly one minute. The fuse is inhomogeneous, and it may burn slowly at first, then quickly, then slowly, and so on. You have a match, and no watch. How do
...

Doable

4
1
Don't Hang Yourself
A fool wants to tie a rope around the earth. So he buys a rope of 40,000 KM and ties it around the world. His neighbour, also a fool, wants to do the same only he wants the rope on sticks 1 meter abo
...

Easy

4
4
Clock Works  Google Interview Question
How many degrees are there in the angle between the hour and minute hands of a clock when the time is a quarter past three?

Doable

2
5
Cracking the Safe
You are to open a safe without knowing the combination. Beginning with the dial set at zero, the dial must be turned counterclockwise to the first combination number, (then clockwise back to zero),
...

Doable

2
3
Happy Handshaking
Jack and his wife went to a party where four other married couples were present. Every person shook hands with everyone he or she was not acquainted with. When the handshaking was over, Jack asked ev
...

Hard

4
3
Balloon Business
You’re sitting in a stationary car. There’s a helium balloon tied to its floor. You accelerate and, obviously feel like you’re being pushed backwards (against the direction of your acceleration). W
...

Doable

4
1
Clock Hands  Google Interview Question
How many degrees are there in the angle between the hour and minute hands of a clock when the time is a quarter past three?

Doable

2
5
The Cabin
There is a man lying facedown, dead, in the middle of a cabin. The cabin is in the middle of the woods. There are windows, but they do not open. What is going on?

Doable

8
All Apples
On the market, mrs. Jones and mrs. Smith sell apples. Mrs. Jones sells her apples per two for 0.50 euro. The apples of Mrs. Smith are a bit smaller; she sells hers per three for 0.50 euro. At a certa
...

Doable

3
2
Open Water
A man is in a rowing boat floating on a lake, in the boat he has a brick. He throws the brick over the side of the boat so that it lands in the water. The brick sinks quickly. The question is, as a r
...

Doable

14
12
Swap Variables in Place without XOR
Suppose you have two variables, a and b. How can you swap their values without using a temporary variable and also without using the bitwise operator XOR?

Doable

1
7
Good luck at an interview with Google
I just came across these Google interview questions… impossible:
How many golf balls can fit in a school bus?
How much should you charge to wash all the windows in Seattle?
How many piano tuners are
...

Hard

5
8
Psycho Killer
While at the funeral of her mother, she met a man whom she did not know. She thought he was amazing, her dream guy, and she fell in love with him but never asked for his number and could not find him
...

Doable

3
2
Surgery
A boy and his father get into an enormous and unfortunate car accident. The father is taken to one hospital and the son to another. The boy’s surgeon walks into the hospital room and says “I cannot
...

Doable

4
Cards With Crosses
You have a stack of 23 cards. Each card has an image of a cross on one side, and an image of a circle on the other side. You know that 14 cards in the stack are laying with the cross up, and therefor
...

Doable

7
Attic Lamp Switch
A downstairs panel contains three onoff switches. One of them controls the lamp in your attic. Using only ONE trip up to the attic, how do you determine which switch controls the lamp?

Doable

1
2
Greatest Area
A farmer challenges an engineer, a physicist, and a mathematician to fence off the largest amount of area using the least amount of fence.
The engineer made his fence in a circle and said it was th
...

Doable

1
5
Right or Left?
This is interesting. Look at the animation below.
Do you see the girl spinning clockwise or counterclockwise

Doable

3
2
Up and Down
Explain the following scenario:
A man lives on the twelfth floor of an apartment building. Every morning he takes the elevator down to the lobby and leaves the building. In the evening, he gets int
...

Hard

16
2
3 Men and a Hotel Room
Three men decided to split the cost of a hotel room. The hotel manager gave them a price of $30. The men split the bill evenly, each paying $10, and retired to their room.
However, the manager real
...

Doable

2
3
The Easy Sum of 1..100?
Quick,without using a calculator, what is the sum of all numbers between 1 and 100 inclusive?
Or 1..10 for that matter..

Easy

0
1
Plane In The Wind
An airplane flies in a straight line from airport A to airport B and back. It has constant engine speed and no wind.
Will its total flight time increase, decrease or stay the same if throughout bot
...

Doable

8
5
Prisoners and hats
100 prisoners are standing in a line each 1 seeing the back of the ones in front of him. Each prisoner is given a black or a white hat. If they can guess the color of their hat they can go away freel
...

Hard

1
2
Stuck in a brick building
A man is stuck inside an all brick building (floor, ceiling, and walls are all brick), that has no windows and no doors, and inside with him he has only a wooden board and a saw, how does he get out?
...

Doable

3
1
Its waistline is a hexagon
Heart of Darkness,
Coat of Grain,
Was once alive,
Now feels no pain,
Never walks, but
Leaves a Trail.
Circle head and pointed tail.
Grows shorter as the day goes on
Its waistline is a hexagon.
Wha
...

Doable

3
What am I?
The first person makes me and sells me.
The second person buys me but doesnt use me.
The third person uses me but doesnt see me.
What am I?

Hard

4
3
Birthday Problem (Classic)
A classic problem.
How many people do you need to have in a room so that the probability of at least 2 of people sharing a birthday is greater than 50%.
Assume 365 days in a year (no leap years)
...

Hard

1
The fisherman and the Hunter
The are two men that always tell the truth except for the days when they are lying. The fisherman lies on Monday, Tuesday and Wednesday. The Hunter lies on Thursday, Friday and Saturday. Today the
...

Easy

4
2
Sold!
A dealer bought a painting for $70, sold it for $80, bought it back for $90, and sold it again for $100.
How much profit did he make?

Easy

3
The Classic Weights & Scale
You have 8 lead marbles, one of which is heavier than the rest. What is the minimum amount of “weightings” (i.e. with a seesaw scale) you need to do to guarantee finding the odd one out?

Doable

0
2
The Twiddled Bolts
Two identical bolts are placed so that their helical grooves intermesh. If you move the bolts around each other as shown in the picture, would they
a) move inward
b) move outward
c) remain the same d
...

Doable

1
3
Ball Drop
A man is floating in a hot air balloon off the coast of Ireland.
In one hand he has a bowling ball, and in the other a golf ball, and he drops them both over the side at the same time.
which one
...

Doable

7
7
Interview Question  Databases
This is a common interview question at companies like Google and Amazon:
“Explain a database in three sentences to your eightyearold nephew.”
This question has an infinite number of solutions. Fee
...

Easy

3
1
Are the lines parallel?
Are the horizontal lines in the picture parallel?

Easy

4
3
The Female Star
My first two letters are a man. The first three a woman. The first four are a brave man. And in whole I am a brave woman.

Doable

5
5
Betsy and Patsy
“We were born on the same day of the same year,” said Betsy.
“And we have the same mother and father,” said Patsy.
“But we’re not twins!” the two exclaimed.
Can you explain?

Doable

22
23
12 Identical Balls
You have 12 identical balls. One of them is slightly different in weight than the rest. You have three weighs of a balance scale to determine which ball is different and whether it is heavier or li
...

Doable
