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.