Massive Algorithms: Answer to Puzzle #40: 4 and 7 minute egg timers or hourglasses to measure 9 minutes

40. Using only a 4 minute and 7 minute hourglass or egg timer how would you measure exactly 9 minutes?

We need to add 2 minutes on to the end of the 7 minute hourglass. Since the twice-flipped 4 minute hourglass will stop at 8 minutes we can flip our 7 minute hour glass here for the second time. At this point it will have been running for 1 minute, we will flip 1 minute's worth of sand back to the top and it will run for another 1 minute. This will total 9 minutes. As shown:
hourglass flipping chart 2