This is a old one, but it is rare to find the general solution: Consider a string (a fuse) that, when burned from one extreme, takes one hour to burn, but irregularly (some pieces burn faster than another). Get two strings of this kind. The irregularities are different, you are only granted the one hour time for each whole fuse. 1) How do you measure 15 minutes using both strings? Spoiler HINT: you can always access the whole string. HINT2: yes, I know the original question asks for 45 minutes. It is the same question, moving the origin of time. But now go to question 2 2) Spoiler And 15 minutes with only one string? 3) Give general formulae for all the time intervals you are able to measure.