Measuring Problem Solution

A teacher of mathematics used an unconventional method to measure a 15-minute time limit for a test. He just used 7 and 11-minutehourglasses. During the whole time he turned sandglasses only 3 times (turning both hourglasses at once count as one flip).
Explain how the teacher measured 15 minutes.

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1. Turn them both over so they start to spill
2. when the 7 min timer is up, turn that one back over. there will be 4 mins left in the 11 min one
3. when the 11 min timer is finished, in the 7 timer it will be 3 mins at the top and 4 at the bottom. You need 4 more mins, so you turn it back over so the 4 mins (in the 7) will spill back.
so it will be the 11 mins (from the 11 timer) and the remainder 4 mins (from the 7 timer)