Imagine the glasses on a swivel
, turning them horizontal stops them.
Use the 'seven' to time the 'eleven'
ie stop the 'eleven' leaving four mins sand left (7><4)
Then use that four mins to measure the 'seven'
ie after four minutes, the 'seven' is seperated into four and three minutes. (4><3)
Now you can do the 15 minutes
Use the four minutes, then the 'eleven'