Tagged Interview Question: If you have a 7 minute hour g... | Glassdoor

## Interview Question

Software Developer Interview(Student Candidate) San Francisco, CA

# If you have a 7 minute hour glass and a 11 minute hour

glass how do you measure exactly 15 minutes

0

2 times 11 - 7 = 15

Anand on Mar 30, 2012
2

launch both hourglass in the same time. When the 7 min is done I start counting. At the end of the 11 min, I have 4 min. I turn upside down the 11 min. At the end of the second round I have 15 min.

Yann on Apr 18, 2012
3

Start both at the same time. When the 7-glass has finished, flip it over immediately. When the 11-glass has finished, the 7-glass will be 4 minutes in, so flip it over again. Once the 7-glass finishes those 4 minutes, 15 minutes total will have elapsed.

This has the advantage of avoiding the 7 minute initial delay, at the cost of being slightly more complex.

x on Sep 9, 2012
0

http://www.cut-the-knot.org/hg_solution.shtml

noob on Oct 21, 2013
0

Start filling 11 min glass first. When it has finished, pour it to the 7 minute glass and pour it back to 11 minute glass. That way you have 7 minute of water in your 11 minute glass. When it is done, it is 15 minutes.

11-7 = 4 and the first 11 => 11+4 = 15

Anonymous on Sep 5, 2014