HKN : discover your destiny

There are 8 iron balls. All are of equal weight except one. And you are given a weighing balance. What is the minimum no of times you need to use the weighing balance to find the heavier ball?

Ans : 2 times

Three times is what is the most common response you’ll come across.

But you can spot the heavier one by using the balance just two times.

Keep three balls on each side of the balance.

Case 1: Both the sides are equally balanced. Which would mean that heavier ball is one of the two left out. Now put each of those two on either side of the balance. One of them would be heavier and the corresponding side would be lowered.

So, for case 1, you can spot the heavier ball by using the balance only two times.

Case 2: Both the sides are not equally balanced. This would mean that the heavier ball is one amongst the three balls, for which the balance is lowered. Next task is to spot which of the three is the heavier one.

So for this, take any two of the three and place them on the scale on either side.

If both of them are of same weight (i.e the balance stays balanced), the heavier one is the third ball which has been kept out.

Else if the heavier ball is one of these two, the corresponding side will be lowered.

So, in case 2 also, we can spot the heavier ball in just two attempts.

Hence, minimum no of times you would need to use the balance is 2.