An Experimental Professor

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 few times. 


Measure 15 minutes usnig 7 & 11 minutes hourglasses

How did he measure the 15 minutes?


Couple of methods to count 15 minutes. 

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