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The Hourglass

You have two hourglasses,
a 4-minute glass and a 7-minute glass.
How do you measure 9 minutes?

Start both hourglasses.
When the 4-minute glass runs out, turn it over (4 minutes elapsed).
When the 7-minute glass runs out, turn it over (7 minutes elapsed).
When the 4-minute glass runs out this time (8 minuts elapsed),
the 7-minute glass has been running for 1 minute.
Turn it over once again.
When it stops, 9 minutes have elapsed.