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### Hour Glass Puzzle

Hour Glass Puzzle - 29 October

I have two sand hour glasses:
1. A 7 minute one and
2. An 11 minute one.

Using just these 2 sand hour glasses, how can i measure time as 15 minutes ?

1. 0:00
Flip both hourglasses

7:00
The 7 min hourglass runs out. Flip it back over.
The 11 min hourglass has 4 minutes remaining. Let it keep running.

11:00
The 7 min hourglass has 3 mins left. Flip it over, and it has 4 mins.
The 11 min hourglass is now empty. Leave it.

15:00
The 7 min hourglass runs out.

2. Start both hourglasses together. When 7 min hourglass runs out (and the 11 min hourglass has four minutes left), start counting time from minute 0.

Now, 11 min hourglass will run four 4 minutes, then flip it over and let it run for its full duration of 11 minutes. This will measure 4 + 11 = 15 minutes.

3. Start with both 11 and 7 glass hors. By the time 11 glass hour completes one round 7 would have completed one round in a direction and 4 in the other direction.

So by the end of 11th minute, in one sand glass we have completed 11minutes and 4 minutes in other.Now reverse the sand glass that completed 4 minutes and it takes 4 minutes to empty out the upper portion

As a whole one sand glass counts 11 minutes and the other counts 4 minutes. That sums up to 15 minutes...

4. 1. Start both the 7 minute hour glass & 11 minute hour glass.
2. Wait till the 7 minute hour glass times out. Time is 7 minute!
3. Restart the 7 minute hour glass. At this time 11 minute hour glass will have 4 minutes left to time out.
4. As soon as 11 minute glass times out invert the 7 minute hour glass. Total time now is 11 minutes.
5. After inverting 7 minute hour glass, it will now have 4 minutes left for time out.
6. After these 4 minutes times out, the total time is 15 minutes.