How will you measure 15 minutes using two hourglasses of 7 minutes and 11 minutes respectively?
Let the hourglass with 7 minutes timeframe be x and the hourglass with 11 minutes timeframe be y.
Turn x and y together.
After seven minutes, turn x.
After 11 minutes, turn x again.
Now it will take 4 minutes for x to stop.
At that moment, you would have calculated 15 minutes.
