Interview Question

Quality Assurance Engineer Interview

-Toronto, ON


You have 9 marbles which are all the same weight, except for one which is slightly heavier than the others. You also have an old?style balance, which allows you to weigh two piles of marbles to see which one is heavier (or if they are of equal weight). What is the fewest number of weighings that you can make which will tell you which marble is the heavier one ?

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The fewest number of weightings is 2. Divide 9 marbles in 3 groups of 3. Weight #1A: 3 in each side. If balance, Weigtht #2A:1 in each side. If balance then the non-weighting is the one, otherwise the heavy side is the one. Weight #1B: 3 in each side. If not balance, then the heavy one has a defect one Weight #2B: Same as in #2A

Dominator on


in 2 weightings you will definitely get an answer. getting answer in one weighing is not always a possibility

Anonymous on


Total 2 steps: Let's divide all marbles in 3 groups A (1,2,3), B (4,5,6) and C (7,8,9). Compare A (1,2,3) with B (4,5,6).. if it's balanced then heavy marble is in group C (7,8,9). Else Either A is heavy or B. In short after 1st test we get the heavy group. Now we have to find from the heavy group (which are 3 marbles). for instance if A group is heavy then Compare marble 1 and 2 --> if balanced then 3 is the heavy one else heady one is already down 1 or 2. Cheers.

Patel on


I disagree on the above, you can divide 9 marbles into three groups of: 4, 4 and 1. Then, weigh 4 and 4 together. If the balance is "balanced", it means the one left aside is the heaviest marble. So, you could accomplish the task in just one weighing, but the chances of that are pretty low. However, that is what the question asks, in my opinion.

Weight-lifter on


As little as one, up to three. 4 and 4 on each side. If it is balanced, you found the heaviest one. If unbalanced, take the heavier group and divide it into 2 and 2, then 1 and 1.

puzzle master on

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