The Sands of Time

From Howard Dinesman’s Superior Mathematical Puzzles (2003):

How can you measure 9 minutes using two hourglass-style timers, one that measures 4 minutes and the other 7 minutes?

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Start both timers. When the first one reaches its limit, turn it over. Do the same with the second. When the first one again reaches its limit, 8 minutes altogether have passed, which means that the second timer has been running for 1 minute since you restarted it. Invert that timer, and when the minute’s worth of sand has run out, 9 minutes altogether will have passed. Similar: How can you measure 45 minutes using two fitful hour fuses?