10 Coins Puzzle | Puzzles for Interview - Interview Preparation PDF Download

Introduction 

The scenario is that you are blindfolded, and ten coins are placed in front of you on the table. You are allowed to touch the coins, but you can't tell which way up they are by feel. You are informed that there are five coins head up and five coins tails up, but you aren't told which ones are which. The puzzle is to make two piles of coins, each with an equal number of heads up.

Solution

Make two piles with an equal number of coins. Now, flip all the coins in one of the piles.
To understand the solution, let's consider a simple case:
P1: H T T T T
P2: H H H H T
In this example, we have five coins with heads facing up (H) and five coins with tails facing up (T). We divide them into two piles - P1 and P2 - with five coins each. P1 has four tails facing up, and one head facing up, while P2 has four heads facing up and one tail facing up.
Now, we flip all the coins in P1. After flipping, the coins in P1 will have four heads facing up and one tail facing up. The new configuration of P1 and P2 is:
P1: T H H H H
P2: H H H H T
As we can see, both piles now have an equal number of heads facing up. Therefore, we have successfully solved the puzzle.

Conclusion

The solution to the puzzle is to divide the coins into two equal piles, and then flip all the coins in one of the piles. This will ensure that both piles have the same number of heads facing up.