Sand Reckoning

Using only a 4-minute hourglass and a 7-minute hourglass, how can you measure 9 minutes?

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Turn over both hourglasses. When the 4-minute glass has run out, turn it over. When the 7-minute glass finishes, turn it over and let it run until the 4-minute glass has run out. At this point we’re at the 8-minute mark and the lower bulb of the 7-minute glass contains 1 minute’s worth of sand. Turn it over to measure the final minute.