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In your interview at a rope pyrochronography factory, they ask you how to time 45 minutes given two ropes that each take an hour to burn.
(The ropes burn uneveningly and unequally. That is, half a rope won't necessarily burn in half an hour, and it won't necessarily be same amount of time as half of the other would take to burn - all you know is that the total time for one rope to burn is one hour.)