There are 2 sand hourglasses.

The small one can measure 5 hours and the large one can measure 7 hours.

How can we measure 16 hours with 2 sand hourglasses running together ?

Steps:

* Start both sand hourglasses at the same time.

* You flip over the small sand hourglass when it finishes the first 5 hours.

* Flip over the large one when it finishes its first 7 hours.

* When the small sand hourglass runs to 10 hours, flip over the small and large sand hourglass at the same time. It means the large sand hourglass will run 3 hours again. When the large sand hourglass run to 13 hours (7 + 3 + 3), flip both the small and large sand hourglass over again to run another 3 hours