Puzzle: Given two hourglass of 4 minutes and 7 minutes, the task is to measure 9 minutes.

Solution:

1. At 0 minutes: Start both hourglasses at the same time.
   
2. At 4 minutes: 4 minutes hourglass runs out and flip it. 7 minutes hourglass is left with 3 minutes.
   
3. At 7 minutes: 4 minutes hourglass is left with 1 minute. 7 minutes hourglass runs out and flip it.
   
4. At 8 minutes: 4 minutes hourglass runs out and 7 is filled with 6 minutes and 1 minute on the other side. Flip it as the sand is left with 1 minute.
   
5. 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