# Two burning ropes measure 5 minutes

Today I was at a job interview and the last question was, "How can I measure 5 minutes by lighting two ropes that each take an hour to burn but do not burn uniformly?" (classical).

I know how to measure 15 minutes and 45 minutes, but 5 minutes....?

• can you cut or burn the ropes? – cats Nov 17 '16 at 22:07
• you can cut and burn them if you want, but burning is not uniform as usual. – user32000 Nov 17 '16 at 22:08
• Does one rope burn for 15min and the other 45min? This isn't clear in the question... – aznbanana9 Nov 17 '16 at 22:10
• This answer seems to imply that it is impossible. It states that you can only measure time intervals that can be expressed as $\frac{m}{2^n}$ hours for integers $m$ and $n$. 5 minutes is $\frac1{12}$ of an hour, an dyou can't express $\frac1{12}$ as $\frac{m}{2^n}$, so it seems that your task is impossible. – GentlePurpleRain Nov 17 '16 at 22:13
• You light one of the ropes, and use the light from it to look at your watch. – Sconibulus Nov 17 '16 at 22:28