You have two ropes and a lighter. Each rope has the following property: If you light one end of the rope, it will take one hour to burn to the other end. They don't necessarily burn at a uniform rate. How can you measure a period of 45 minutes?
Both the ropes take longer than 45 minutes to burn from one end to other. So we need to speed up the process to measure the 45 minutes.
Light both ends of rope A and one end of rope B. After 30 minutes, rope A will be completely burned up and there will be 30 minutes of rope B left. Light the other end of rope B; it will burn up in 15 minutes. Total time elapsed since starting the ropes on fire: 45 minutes.