You have a lighter and two fuses that take exactly one hour to burn, but they don’t burn at a steady rate. For example, one fuse could take 59 minutes to burn the first inch and then burn the rest of the fuse in the last minute.
How would you use these two fuses to measure 45 minutes?
Answer: Light the first fuse on both ends and the second fuse at only one end. When the first fuse burns out you know 30 minutes have passed. Light the other end of the second fuse and when it burns out, 45 minutes have passed.