#1 - Tricky Probability Interview Puzzle

I have two coins.
* One of the coin is a faulty coin having tail on both side of it.
* The other coin is a perfect coin (heads on side and tail on other).

I blind fold myself and pick a coin and put the coin on table. The face of coin towards the sky is tail.

What is the probability that other side is also tail ?

Tricky Probability Interview Puzzle


2 possible scenario are :
case number side shown other side
1 A1 (H) A2 (H)
2 A2 (H) A1 (H)
3 B1 (H) B2 (T)
4 B2 (T) B1 (H)

case 4 is not possible so ans is 2/3

#2 - Logical Thinking Riddle

I have 100 coins in my wallet.

What is the minimum number of coin(s), i would be required in order to make sure each coin touched exactly three other coins.

Logical Thinking Riddle


3 placed flat on the table in a triangle(touching each other) and put the fourth one on top of them in the middle.

#3 - Challenging Logic Puzzle

In front of you, there are 9 coins. They all look absolutely identical, but one of the coins is fake. However, you know that the fake coin is lighter than the rest, and in front of you is a balance scale. What is the least number of weightings you can use to find the counterfeit coin?

The answer is 2. First, divide the coins into 3 equal piles. Place a pile on each side of the scale, leaving the remaining pile of 3 coins off the scale. If the scale does not tip, you know that the 6 coins on the scale are legitimate, and the counterfeit is in the pile in front of you. If the scale does tip, you know the counterfeit is in the pile on the side of the scale that raised up. Either way, put the 6 legitimate coins aside. Having only 3 coins left, put a coin on each side of the scale, leaving the third in front of you. The same process of elimination will find the counterfeit coin.

#4 - Smart Maths Bet Puzzle

Two friends were betting. One said to the other, “The coin will be flipped twenty times and each time the coin lands on head, I will give you $2 and each time it lands on tale, you will give me $3.” After flipping the coin for twenty times not a single penny was exchanged among them.

How many times did the coin land on heads ?

Smart Maths Bet Puzzle


Let x be the number of time heads appeared
2x = 3(20-x) --- this is because the amount won and lost was equal

#5 - Coin Riddle

If you were to put a coin into an empty bottle and then insert a cork into the neck, how could you remove the coin without taking out the cork or breaking the bottle?

Coin Riddle

Simply push the cork into the bottle and shake the coin out.