Math Problem.

Recently I ran into this problem in the M. Gardner's Mathematical Circus.
 What is the quickest way to measure 9 minutes with a 4-minute hourglass and a 7-minute hourglass?
Assuming you are unable to measure fraction intervals; ergo you cant measure 2 minutes.

>>1
It didn't come with a solution?
Start both at the same time, turn the 4 when it empties. When 7 is empty, use 4 to measure 1 + 4 + 4 = 9.
Total time taken: 7 + 9 minutes. Invest in a digital stopwatch.