On Friday I posted this puzzle….
You have two ropes and some matches. Each rope, if lit at its end, will burn for 60 minutes. But the rate of burning is not regular, so cutting a rope in half doesn’t result in a burn time of 30 minutes.
How can you use the ropes to time exactly 45 minutes?
If you have not tried to solve it, have a go now. For everyone else the answer is after the break.
Light both ends of the first rope, and one end of the second rope, at the same time. After 30 minutes, the two burning ends of the first rope will meet, and the second rope will have 30 minutes left to burn. At that point, light the remaining end of the second rope. 15 minutes later the two burning ends of the second rope will meet, and you will have timed 45 minutes.
I have produced an ebook containing 101 of the previous Friday Puzzles! It is called PUZZLED and is available for the Kindle (UK here and USA here) and on the iBookstore (UK here in the USA here). You can try 101 of the puzzles for free here.