Multiple choice

You have nine balls. All weigh the same, except for one which is heavier. You have a weighing balance. What is the minimum number of weighings required to pick the heavy ball?

  1. 1

  2. 2

  3. 4

  4. 5

  5. 7
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Take 6 balls and keep 3 each in each pan. Now, if 1 of these balls is the heavier one, select the 3 balls of which one is the heavier one. Take 2 balls out of these 3 and keep them in the balance, 1 in each pan. If these 2 are of equal mass, then the 3rd ball you left out is the heavier one. Else, the heavier ball out of the 2 is the heavier ball you wanted.  Suppose all the 6 balls you picked were of equal mass. Then 1 out of the 3 balls you left out is the heavier one. Now, as before, you can identify the heavier ball by doing just 1 weighing.  Hence, the number of weighings you have to do is just 2.

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