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.
There were 2 ways for professor to count 15 minutes using 11 & 7 minutes hourglasses.
Method 1 :
1. He started both 7 & 11 minutes hourglasses but didn't begin test.
2. After 7 minutes hourglass ran out, he started the test. Still 11 one yet to count 4 minutes.
3. After conducting test for 4 minutes, 11 hourglass ran out. 4. Now he turns around 11 hourglass & continues. So 11 + 4 = 15 minutes counted.
Method 2 :
1. He started both the hourglasses & started to conduct the test as well.
2. When 7 minutes ran out he turned around it & kept 11 minutes counting.
3. After 4 minutes, 11 minutes hourglass ran out. Meanwhile 7 minutes hourglass also counted 4 minutes. So far 11 minutes counted. 4. Then he again turned around 7 minutes hourglass which had counted 4 minutes. That's how 7 minutes counted 4 minutes again. 5. In this way, 11 + 4 = 15 minutes counted.
