The time difference between both the hour glasses is 4 minutes and we should make use of this time and add it to 11 minutes hour glass time to get 15 minutes.
At time 0 minutes:
hour glass1 - 0-11
hour glass2 - 0-7
At time 7 minutes:
hour glass1 - 7-4
hour glass2 - 7-0
Invert hour glass2.
At time 11 minutes:
hour glass1 - 11-0
hour glass2 - 4-3
Again invert hour glass2 to make use of the 4 minutes difference.
At 15 minutes:
hour glass1 - 11-0
hour glass2 - 0-7
When hour glass2 is emptied 15 minutes would pass.
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