Question: Measure 9 minutes from a 4 minutes hourglass and a 7 minutes hourglass?
At 0 minutes: start both hourglasses at the same time.
At 4 minutes: 4 minutes hourglass runs out and flip it. 7 minutes hourglass is left with 3 minutes.
At 7 minutes: 4 minutes hourglass is left with 1 minute. 7 minutes hourglass runs out and flip it.
At 8 minutes: 4 minutes hourglass runs out and 7 is filled with 6 minutes and 1 minute in other side. Flip it as the sand is left with 1 minute.
At 9 minutes: 7 minutes hourglass becomes empty from above side.
Hence we measured 9 minutes.
This article is contributed by Vikash Kumar. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above
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