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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.
How did he measure the 15 minutes?
Couple of methods to count 15 minutes.
Source
What was the challenge?
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.
