Hourglasses 2 Puzzle - Solution
The Puzzle:
An eccentric professor used a unique way to measure time for a test lasting 15 minutes.
It needed two hourglasses: one measured 7 minutes and the other 11 minutes.
During the whole time the hourglasses were turned only 3 times.
How was the 15 minutes measured?
Our Solution:
When the test began, the professor started both hourglasses running.
When the 7-minute hourglass ran out, it got turned around.
4 minutes later, the 11-minute hourglass ran out, and the 7-minute hourglass was promptly turned around again, so the 4 minutes ran back again.
11 + 4 = 15, and the test was over.
Footnote: Dr. J Sreedhar wrote to tell me that the Prof could have done it with one less flip:
• Start both (11-minute and 7-minute) hourglasses, but not the test.
• When the smaller one runs out, start the test. The bigger hourglass has 4 min to go.
• When the bigger hourglass also runs out, just flip it to measure out 11 more min.
• Test is over when the bigger hourglass runs out for the 2nd time.