I was recently called for an interview, and the interviewer asked me a puzzle. It goes something like this:
"You have two containers, C1 and C2, both filled with equal volumes of oil and water, respectively (i.e., C1 -> oil volume V and C2 -> water volume V). A spoonful of liquid is taken from C1 and poured into C2, then a spoonful of liquid is taken from C2 and poured into C1. After these two steps take place, which container will have higher purity (concentration of oil for C1 and that of water for C2)?"
I thought for a while and answered C1, citing that since we put pure oil from C1 into C2 but a mix from C2 into C1, certainly some oil from C2 is back in C1, thereby achieving more purity. The interviewer said it was wrong and asked me to try again. This time, I did some actual math and was told C2 would have a greater concentration based on the calculations. The interviewer again tagged me wrong, so I asked the interviewer to explain, and his answer was that both would be equal since you take 1 spoon of oil from C1 to C2, and put 1 spoon of water from C2 to C1. This is obviously flawed, as no oil is getting back on the spoon when you dip it into C2. I wanted to point that out but didn't. Later, I re-did my math thoroughly and found that C2 would have a greater purity of water.
