View Answer NowStart both 7 min & 4 min hour glass at the same time. Once 4 hour glass completes then reverse it; now 7 hour glass is at 4 min and 3 min remaining. When 7 hour glass completes then reverse it. the reversed 4 hour glass still has 1 min to complete. After you reverse 7 hour glass wait until the 4 hour glass to be completed thereby your total is 9 min.