Sand Timer Puzzle
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You have two sand-timers, one of 11 min and another of 7 min.
How will you measure 15 min using them?
Start both the timers viz. 7min & 11 min.
When 7min timer completes, just reverse it so that it starts again..
When 11min completes, just reverse the 7min timer again (4 min would have lapsed = 11 – 7)
Wait for 7min timer to complete; it will take 4 min… totalling 11 + 4 = 15 minutes!
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