Two Hourglasses puzzle
You have two hourglasses.
One contains seven minutes of sand and
the other contains eleven minutes of sand.
Using just these two hourglasses,
how can you accurately time fifteen minutes?
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Answer :First start both the hourglasses
but dont start counting time of 15min.
Now when the 7min hourglass finishes,
start counting time.
So now you will have 4 minutes left in 11 min hourglass.
After this 4min are over flip the hourglass and let it run its full 11mins
So you will get 4 + 11 = 15 minutes.