How can you accurately time 15 minutes?
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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.
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.
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bharathraj
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