This question has actually been asked at job interviews (Microsoft, for example) and on math final exams in college.
It’s fairly difficult.
There are two lengths of rope.
Each one can burn in exactly one hour.
They are not necessarily of the same length or width as each other.
They also are not of uniform width (may be wider in middle than on the end), thus burning half of the rope is not necessarily 1/2 hour.
By burning the ropes, how do you measure exactly 45 minutes worth of time?
Feel free to post your answer in the comments section. Don’t look, though, until you really give it a try!