How can you accurately time 15 minutes?

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If you have an 11 minute and 13 minute hourglass, how can you accurately time 15 minutes?

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start A and B at a time.
A =11,B=13
After  A completed Reverse A.
After B completed in A 2 minutes will be recorded.Stop B and turn A one more time u will get 15 minutes.
1.A=11,B ==13
2.A=11
3.A=2.B=13
4,A=2+B

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on 19th January 2016.
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  • 2 Answer(s)

    Start both hourglasses then when the 11 minute hourglass has finished immediately flip it again. When the 13 minute hourglass runs out the 11 minute hourglass will have 9 minutes left, so flip it and it will last another 2 minutes, 13 minutes + 2 minutes = 15 minutes.

    silent47 Expert Answered on 22nd August 2015.
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    Start both hourglasses. when 11 hourglass has finished, this is the time which is beginning of 15 minutes . when 13 hourglass has finished then flip it immediately again.  when 13 hourglass has been finished again we get (13-11)+13 = 15 minutes.

    vdmohit Starter Answered on 4th September 2015.
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