Solution:
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
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