# Hourglasses 2 Puzzle - Solution

## The Puzzle:

## 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.