Timing Puzzle 1
Having 2 sand-glasses: one 7-minute and the second one 4-minute. How can you time correctly 9 minutes ?
Timing Puzzle 2
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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