Câu hỏi:
If you have two hourglasses -- one four-minute timer and one seven-minute timer, how can you measure nine minutes?
Đáp án:
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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