Start both hour glasses as the egg is dropped in the boiling water. When the sand stops running in the 7-minute hourglass, turn it over. When the sand stops running in the 11-minute hourglass, turn the 7-minute hourglass over and leave the 11-minute hourglass. When the sand stops in the 7-minute hourglass, 15 minutes will have elapsed.
PUZZLE #4 - THE OLD LADY WHO WITNESSED SOMETHING
The murdered was counting which level the old lady was at....so he could come and KILL HER!