Logic Puzzle

You have two ropes, each of which takes exactly 1 hour to burn from end to end. Each rope has different material densities at different points, so there's no guarantee how long it will take different sections within the rope to burn—for instance, the first half might burn in 59 minutes and the second half in 1 minute. You need to find a way to measure exactly 45 minutes by burning these two ropes.

Answer to the last Logic Puzzle:

Take one coin from stack 1, 2 coins from stack 2, 3 coins from stack 3, etc. Take a single measurement of the 55 coins. The stack is the amount it's off from 55.0 grams / 0.1 grams. So, if it's off by 0.1 grams (54.9 grams), stack 1 contains the 0.9 gram coins. If it's off by 0.2 grams, it's stack 2, and so on.