DAY 124

Two bicycle riders are 60 miles apart. They begin riding directly toward each other. At the same instant, a fly leaves the front tire of one of the bicycles, flies directly to the other bicycle's front wheel, instantly turns around and flies back to the first bicycle, and continues this until the bikes meet and the fly is unceremoniously squished. If the bicycles travel at 30 mph and the fly flies at 15 mph, how far will the fly have flown when it is squished?

Answer:

15 miles.........The two bicycles are approaching each other at a combined speed of 60 mph. This means it will take them one hour to meet. The fly will fly 15 miles in that hour.

DAY 125
How can you cook an egg for exactly 15 minutes if all you have are two uncalibrated timers, one that runs for 7 minutes and one that runs for 11 minutes? (Think of the timers as sand hourglasses.)

Answer:

Start both timers at the same time. When the 7-minute timer runs out, start cooking the egg. When the 11-minute timer runs out, the egg will have been cooking for 4 minutes. Immediately turn over the 11-minute timer to give you 4 + 11 = 15 minutes.

Alternate solution:
Credit GAMES Magazine
November, 1983
Steve Odell

Start cooking the egg and turn over both timers at the same time. When the 7-minute timer runs out, turn it over to start timing again. When the 11-minute timer runs out, the 7-minute timer will have been running for four minutes. Turn the 7-minute timer over again. After another four minutes, the 7-minute timer will run out. 11 minutes + 4 minutes = 15 minutes.
(posted 2/17/16)