Start both hourglass at same time

As 7minutes gets competed,by 11 minutes

We remains with 4 minutes

Now as soon as 11 minutes hourglass completes ,reverse it

We get 11 minutes

Finally We get 15 minutes,4 minutes and this 11 minutes

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How will you measure 15 minutes using two hourglasses of 7 minutes and 11 minutes respectively?

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