Burning Ropes
Riddle: You have two ropes of uneven width that each take an hour to burn out (if you have trouble visualising this just know that 30 min won't be exactly in the middle, and 20 min won't be a third, etc.). How do you measure 45 min with these two ropes and fire?
Answer: You set fire to both ends of one rope but only one end of the other. When the first rope burns out it will have been 30min. When this happens, you set fire to the other side of the other rope. After that burns out, in 15min, it will have been 45min.
Source: https://www.riddles.com/6750
