I had an interesting puzzle during a recent interview. I am paraphrasing this from memory so forgive me if you know this one and I am misquoting it somehow:
You have two types of pebbles, white and black, and three jars labeled white, black and black & white, respectively. One jar contains all white pebbles, one contains all black pebbles and one contains a mixture of black and white pebbles. The three jars a all mislabeled so that they will not contain the pebbles noted on the label. How many pebbles would you have to draw and from which jars in order to determine the true distribution?
Don’t read any farther if you are going to try and solve this one for yourself. The next paragraph contains the answer.
The answer is one, from the jar labeled “black & white”. If you got it on your first try through, congratulations, I did not. I figured it out on my second run through it.
Basically, you draw one pebble from the “black & white” jar. Say you draw a white one, you then know that the jar labeled “black & white” is the jar containing the white pebbles. This leaves you with two unknown jars, one labeled “white” and one labeled “black”. Since you know that both of these are incorrect and you have a white pebble, you know that the jar labeled “white” contains the black pebbles and the one labeled “black” contains the black and white pebble mixture.
It’s an interesting problem, but honestly I have never felt that these sorts of problems are useful in a technical interview. We would usually have one question of this nature in our interviews more to see how they would go about solving it than looking for an actual answer. Google apparently loves this type of question in an interview, but asks very little of a technical nature.
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