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Question 1120823: Can you measure a time interval of 7 minutes using two sandglasses of 5 and 8 min each? If so, how?
Answer by ikleyn(53765)   (Show Source):
You can put this solution on YOUR website! .
Setup both sandglasses to the normal start position and let them start simultaneously.
After 5 minutes (measured with 5-minute sandglass) 5-minute sandglass is empty; 8-minute sandglass has 3 minutes remained.
At this moment, turn over 5-minute sandglass; 8-minute sandglass still continue to measure their remaining 3 minutes.
When 8-minute sandglass will empty, 5-minute sandglass still has 2 minutes remaining.
Starting from this moment, you start measure/watch your 7-minute time interval.
You wait these remaining 2 minutes at 5-minute sandglass and then turn it over to wait (to give it) 5 additional minutes.
2 + 5 = 7 minutes is that 7-minute interval you were requested to measure.
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