If you have two hourglasses — one four-minute timer and one seven-minute timer, how can you measure nine minutes? Turn over both hourglasses at once.
When the four-minute hourglass runs out, turn it over.
When the seven-minute hourglass runs out, turn it over.
When the four-minute timer runs out this time (eight minutes have elapsed), the seven-minute hourglass has been running for one minute.
Now turn over the seven minute timer back over. When the timer runs out, nine minutes have elapsed.

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