November 15, 1999
Problem of the Month

The Cooking Problem

You have two egg-timers -- a four-minute timer and a seven-minute timer.
These are hourglasses which contain sand.

Explain how you can cook an ear of corn for 9 minutes.   (Tell when you would turn over the timers, when you would start cooking the ear of corn, when it would be done, etc.).

Solution to the Problem:

There are several solutions to this problem.

Here is one of them:

Start cooking the corn and turn over both timers.

After the 4-minute timer runs out, turn it over.

After the 7-minute timer runs out, turn it over.

When the 4-minute timer runs out the second time, turn over the 7-minute timer (which has 1 minute of sand in it).

After the sand runs out in the 7-minute timer, 9 minutes will have elapsed and the corn will be done.

     7 min                   1 min 1 min 
|----------------------------|----|----|

     4 min          4 min
|----------------|----------------|

              9  min
|<------------------------------------>|           
             

Correctly solved by:

1. Tom Marino	Winchester, VA
2. Jia Ran	Rome, Italy
3. Court Pifer	Winchester, VA
4. Bill Brown	Winchester, VA
5. Chip Crawford	Winchester, VA
6. Jonathan Pence	Winchester, VA

Send any comments or questions to: David Pleacher