Aptitude - Clock Synchronization Puzzle

You are given two highly irregular delay lines (ropes) used in an explosive detonation circuit. Each delay line takes exactly 60 minutes to burn from one end to the other. However, they burn at highly inconsistent rates (e.g., half the line might burn in 10 minutes, and the other half in 50 minutes). Using only a lighter, how can you measure exactly 45 minutes to trigger a precise event?