Hourglasses 2 Puzzle

The Puzzle:

Hourglasses 2
An eccentric professor used a unique way to measure time for a test lasting 15 minutes.

He used just two hourglasses. One measured 7 minutes and the other 11 minutes.

During the whole time he turned the hourglasses only 3 times.

How did he measure the 15 minutes?



.
-\./^\./-


Do you have the answer? Check against our solution!

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