Question

An eccentric professor used a unique way to measure time for a test lasting 15 minutes. She used just two hourglasses. One measured 7 minutes and the other 11 minutes. During the whole time she turned the hourglasses only 3 times. How did she measure the 15 minutes

Answer 1

When the test began, both hourglasses started to run.

After the 7 minutes hourglass ran out, she turned it around.

After 4 minutes from then, the 11 minutes hourglass ran out, and 7 - 4 = 3 minutes are left in the 7 minutes hourglass. Then she turned around the 7 minutes hourglass with 7 - 3 = 4 minutes left. Given that 11 minutes had passed, after these 4 minutes, the test is over.