Problem of the Month - April 2025

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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