Hourglasses 2 Puzzle - Solution
The Puzzle:
He used just two hourglasses. One measured 7 minutes and the other 11 minutes.
During the whole time he turned the hourglasses only 3 times.
How did he measure the 15 minutes?
Our Solution:When the test began, the professor started both hourglasses running.
When the 7min hourglass ran out, he turned it around.
4 minutes later, the 11min hourglass ran out, and he promptly turned the 7min hourglass around again, so the 4 min 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 (11min and 7min) 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.
When the 7min hourglass ran out, he turned it around.
4 minutes later, the 11min hourglass ran out, and he promptly turned the 7min hourglass around again, so the 4 min 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 (11min and 7min) 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.