This “burning rope” problem is a classic logic puzzle. You have two ropes that each take an hour to burn, but burn at inconsistent rates. How can you measure 45 minutes? (You can light one or both ropes at one or both ends at the same time.)

Answer

Because they both burn inconsistently, you can’t just light one end of a rope and wait until it’s 75 percent of the way through. But, this is what you can do: Light the first rope at both ends, and light the other rope at one end, all at the same time. The first rope will take 30 minutes to burn (even if one side burns faster than the other, it still takes 30 minutes). The moment the first rope goes out, light the other end of the second rope. Because the time elapsed of the second rope burning was 30 minutes, the remaining rope will also take 30 minutes; lighting it from both ends will cut that in half to 15 minutes, giving you 45 minutes all together.