[Math Lair] The Two Hourglasses

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Can you think logically and think on your feet? Here is a scenario that tests both:

You are shipwrecked on an island. Unfortunately for you, the natives are cannibals and they have captured you.

The chief gives you a pair of hourglasses. The first one measures exactly four minutes, while the other measures exactly seven minutes. The chief says that you must tell him when exactly nine minutes have passed. If you can do so, you will be allowed to go free. If not, the cannibals will eat you for dinner.

The chief says, "Start now!" What do you do?

The answer is on the answers page.