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