You have two sand timers with you. One can measure 7 minutes and the other sand timer can measure 11 minutes. This means that it takes 7 minutes for the sand timer to completely empty the sand from one portion to the other.
You have to measure 15 minutes using both the timers. How will you measure it ?
Start both the timers together.When seven minutes timer get finished reverse it( count=7 minutes ).
Now wait till 11 minute timer finishes(count=11 minutes).Once 11 minute timer finishes reverse the 7 minutes timer again.Once this finishes we are done.