You have a 7 minute and a 4 minute hourglass timer. How would you use these to measure 9 minutes?
Solution
Flip the 7 minute and 4 minute timer at the same time. When the 4 is done, flip it again. When the 7 minute timer is done, flip it again. When the 4 minute is done, flip the 7 minute timer. When the 7 minute timer is done, 9 minutes will have passed from the first time the timers were flipped.
How would you measure 15 minutes with a 7 and 11 minute hourglass timer?
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