Having 2 sandglasses: one 7-minute and the second one 4-minute, how can you correctly time 9 minutes?

Answers (1)

Answer: when the 4 minute one run out turn it over immediately so it starts counting 4 minute again.

When the 7 minute one runs out , turn it over so it starts counting again

The moment the 4 minutes one runs out for the second time , turn the 7 minute hourglass over

It will have only been running exactly one minute .

Let the sand run back again (1 minute more ) and then straight away , 9 minute

\ 4	imes 2+1=9

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