How do you measure 9 minutes with a 7-minute and a 4-minute hour glass? Assume t

Question

How do you measure 9 minutes with a 7-minute and a 4-minute hour glass? Assume the hourglass can only measure 7-minute and 4-minute intervals and cannot be used to measure other time intervals (for example, your cam tell when 2 minutes have gone by) Choose the correct answer below. Flip over both hourglasses. When the 4-minute hourglass runs out. turn it over. When the 7-minute hourglass runs out turn it over. When the 4-minute timer runs out again it has been going for 8 minutes, the 7-minute hourglass has been running for 1 minute Now turn the 7-minute timer back over. When the timer runs out 9 minutes have gone by. Flip over the 7-minute hourglass When the 7-minute hourglass runs out. turn it over and flip over the 4-minute hour glass When the 4-minute timer runs out it has been going for 8 minutes, the 7-minute hourglass has been running for 1 minute. Now turn the 7-minute timer back over. When the timer runs out 9 minutes have gone by. Flip over the 4-minute hourglass. When the 4-minute hourglass runs out. turn it over and flip over the 7-minute hour glass. When the 4-minute timer runs out again it has been going for 8 minutes, the 7-mmute hourglass has been running for 1 minute. Now turn the 7-minute timer back over When the timer runs out. 9 minutes have gone by. This is not possible. The hourglasses cannot accurately measure 9 minutes.

Explanation / Answer

Since the hourglass can only measure 7 and 4 minute intervals we cannot measure 9 minutes.

7-4 = 3 minutes and the difference can be measure.But 9 minutes cannot be done by any means !

Correct Answer is D.

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