You have a lighter and two lengths of rope that burn for exactly 1 hour each. They do not burn consistently so you cannot, for example, cut one in half and burn that. How can you use these two ropes to measure exactly 45 minutes ?
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Light one rope from both ends and one rope from one end.
After 30 minutes, the first rope will be finished burning.
You have 30 minutes left of burning on the second rope. Light the second end of that rope so that it will burn for 15 minutes instead of 30 minutes.
The total burning time will be 45 minutes.