measuring 9 minutes with a 4 minute sandglass and a 7 minute sandglass

Posted: September 27, 2012 in Mathematics

Looking for a job isn’t easy nowadays.  There is a rumor that in Google, they will ask you some questions in the interview to test your problem solving skill.  One of the questions I saw on a forum is:

“How do you measure exactly 9 minutes with only a 4 minutes sandglass and a 7 minute sandglass in your hand?”

I spent about couple minutes in front of the computer while watching “How I meet your mother” online then I come up with my solution.

First start with both sandglass.  When the 4 minute sandglass done dripping, you know 4 minutes are gone.  Now you should have 3 minutes of sand in the 7 minute sandglass, turn the 4 minute sandglass upside down.  Wait until the remaining 3 minutes of sand in the 7 minute sandglass is done, that’s a total of 7 minutes are gone.  While there is 1 minute of sand left in the 4 minute sandglass, you flip the 7 minutes sandglass.  A minute later, that should be 8 minutes in total, the 4 minute sandglass is done.  There should be 1 minute of sand at the bottom of the 7 minute sandglass.  And now you flip that 7 minute sandglass.  At the end the 7 minute sandglass should finish in 1 minute, which is 9 minutes in total.  We are done!

Back to How I met your mother 🙂 I may not be smart enough to get a job at Google.

Ciao

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