Timing 9 minutes

Step 1: Start both hourglasses simultaneously.
Step 2: When the 4-minute runs out, restart it.
Step 3: When the 7-minute runs out, restart it. (The 4-minute will have 1 minute to go).
Step 4: When the 4-minute runs out, restart the 7-minute. (The 7-minute will have 1 minute to go).
When the 7-minute runs out, the required 9 minutes will have elapsed.

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