In this “episode”, there are two different puzzles, both of them are measuring a specific duration of time with a few “timers”.
Here is the first one:
You have two lengths of fuse, each will burn for exactly one hour. But the fuses are not necessarily identical and do not burn at a constant rate. There are fast burning sections and there are slow burning sections. How do you measure 45 minutes using the fuses and a lighter?
Source: How Would you move Mount Fuji, by William Poundstone; Puzzle #50
Target: 12+ years
Here is the second one:
You have two timers, one that goes off after 7 minutes and another one that goes off after 11 minutes. You need to boil an egg for exactly 15 minutes. How would you measure 15 minutes using the two timers?
Source: Mathematical Cirlces (Russian Experience) by Dmitri Fomin, Sergey Genkin and Ilia Itenberg (Page 67, Puzzle #18)
Target: 8+ year
The 2nd puzzle is quite simple – start both timers together.. When the 1st timer goes off at the end of 7 mins, u start boiling the egg.. Thr are 4 mins left for the 2nd timer to go off (11 – 7 mins).. The moment the 2nd timer goes off, start it again for another 11 mins.. When the 2nd timer goes off for the 2nd time, u’ve got 4 + 11 = 15 mins.. Still thinking on the 1st puzzle, which does not seem as simple..
Got through to the second one but still scratching my head on the first one! Will give it little more thought. Hope all is well. Thanks for making me use my brain 🙂
Good brain teasers, answers in email!