i've got a few questions interviewers like to ask. (yeah, i'm probably quite a bit older than you guys and am working). these aren't overly complicated, but you're under tremendous pressure to solve it within minutes since, well, there's somebody sitting right across you. here's one that's pretty good. i first heard about it from somebody who interviewed for goldman sachs, but it's very well-known--well-known enough that i was asked the same question by a hedge fund i interviewed with later on.
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you have two identical plates. you're standing next to a 100-floor building and your job is to figure out at what height (i.e. floor) these plates will break by releasing the plates from various floors. what is your optimal strategy? (to be exact, minimize the maximum number of trials, but i guess that was implicitly assumed since in my interview the guy didn't specify.)
so, for example, one strategy could be you do something like a binary search. first, you release the plate from the 50th floor. if it doesn't break, you move up to the 75th floor and try again. now if it does, you know the plates break somewhere between the 50th and 75th floor. of course, you're now only left with 1 plate, so to figure out that exact floor, you'll need to test it floor by floor starting from the 51th to the 74th floor. so if the floor is the 75th floor, you'll need like 2+24=26 tests.
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have fun!
one word: don't fret if you don't get it within a few minutes. usually, when the interviewers sees that you're getting stuck, some hints would be coming your way. (obviously it's better not to receive any hints, but some hints is okay).
the Wii is an epidemic.
