How you will measure 9 minutes from a 4 minutes hourglass and a 7 minutes hourglass?

Question

Answer 1

Timing and steps below describe how to achieve 9 mins:

0 min- start both hourglasses
4 min- flip 4 min hourglasses when 4 mins over
7 min- start timing when 7 mins over- 4 min hourglass has 1 min left
8 min- flip 4 min hourglass- got 1 min
12 min- flip 4 min hourglass- got 5 min
16 min- 4 mins over- end timing- got 9 mins