Interview Question

Global Portfolio Planning Interview

-Bloomington, IL

General Motors (GM)

Brain Teaser: you have 7 seemingly identical looking balls, only 1 is slightly lighter than the other 6. You are given a scale. How can you find the 1 lighter ball is you are only allowed to use the scale twice.

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4 Answers


pick 3 random balls on one side of the scale, pick 3 more to put on the other end. if scale equals, the 1 unpicked ball is the lighter one. if one side is lighter than the other one, the lighter ball is among them. take 1 random ball out of 3 to put on one side the scale, put another on the other get it by now.

Anonymous on


can we not pick 2 on each side?

Srinivas Krothapalli on


Add one ball to the scale at a time continuously . The lighter ball will show a slight difference at the scale when you adding up the weight. Like if the first ball is 12g, then adding the next one will be 24, 36, etc and the one that was slightly off will be the lighter one. I think this count as using the scale once because I use it continuously

Vincent on


You can start with 2 or 3 on each side and still only measure twice...doesn't matter. Second step would be to measure 2 of the 3 balls from the lighter side. Regardless, it's a dumb question to ask someone on an interview.

My Opinion on

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