You have two ropes, each of which takes exactly 60 minutes to burn from one end to the other.
The ropes are not identical, nor are they uniform – that is, it’s not guaranteed that one half of the rope takes exactly 30 minutes to burn.
How can you use these two ropes to time exactly 45 minutes?
Here’s one solution to the puzzle:
1. Light the first rope at both ends, and the second rope at one end.
2. The first rope will burn completely in 30 minutes, at which point the second rope will have burned halfway.
3. Light the other end of the second rope. It will take 15 more minutes for the second rope to burn completely, for a total of 45 minutes.
4. When the second rope has burned completely, exactly 45 minutes will have passed.
The key idea behind this solution is that you use the first rope to time 30 minutes (since it burns completely in 60 minutes) and then use the second rope to time the remaining 15 minutes (since it has already burned for 30 minutes, and will take another 30 minutes to burn completely).