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February 18, 2018, 10:55:49 AM
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: Hour Glasses  ( 3012 )
« : May 14, 2007, 02:09:33 PM From Shrinidhi»



Hour Glasses


You have two hourglasses: a 7 minute one and an 11 minute one. Using just these hourglasses, accurately time 15 minutes.

 
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« #1 : May 15, 2007, 04:25:38 PM From Ria»

In such problems we can start measuring either during the start or in a time convinient to us. The question is not clear on when we can start measuring 15 minutes. However, assuming that we start measuring as soon as the hour glasses are modified,
1. Wait till the 7 minute glass is full. (7 minutes)
2. Copy the 7 minute glass content to 11 minute glass and wait till 11 minute glass is full ( 4minutes)
3. Copy the entire content to 7 minute glass and discard the excess 4 minutes collection.
4. Copy back the 7 minute glass sand back to 11 minute glass.
5. Wait till the 11 minute glass is full. (4 minutes)

Totally it will be 15 minutes.
« #2 : May 18, 2007, 04:04:29 PM From Poonam»

Ashish, you are completely wrong.
Hour glasses are not ordinary glasses that you pour contents of one glass into another. They can only be used to measure time.

The answer is as follows:
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.
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