Brain Teaser-2

Katie the cook was in a predicament over a pie. The recipe called
for baking it for exactly nine continuous minutes, but all she had in
the way of clocks were two egg-timers. The first one measured a
period of four minutes, and the second one seven minutes. If there
are no graduations on the egg-timers, and sand cannot be removed or
transferred from them, what is the quickest way that Katie can
measure nine minutes with the two timers?

SOLUTION

Put the pie in the oven and start both egg-timers. When the first one
runs out, turn it over. (Elapsed time so far is 4 minutes.) When the
second timer runs out, turn it over, too. When the first one runs out
again (after a total of 8 minutes), the second timer has been running
for one minute since it was re­started, so turn it over again.
When the one minute runs out, the pie is ready.