9 Minutes with 4 Minute, 7 Minute Hourglass


How to measure 9 minutes using only a 4 minute and 7-minute hourglass?


This question is similar to 4 Quarts of Water. With water, we were able to fill and pour out the pail multiple times. We can use the same logic and easily come up solutions which start measuring 9 minutes somewhere in the middle of the procedure but those will not be ideal. If possible, we want to measure the 9 minutes right from the start.



StepTime4 minute timer7 minute timer
10 minStartStart
24 minsFlip3 minutes left
37 mins1 minute leftFlip
48 minsStopFlip (1 minute left)
59 minsStop