You have two ropes that each burn for 1 hour end to end, but at varying rates.
You have a lighter. You cannot cut the ropes (and doing so wouldn’t help, since they burn at unknown rates and you only know total burn time).
How do you measure 45 minutes?
Answer:
Start the timer.
Burn one at both ends. Burn the other at one end.
When the first is completely burned, start burning the other end of the second.
When the second is burned completely, 45 minutes have elapsed.