Hourglass Problem (Brain Teaser)

Here is famous Hourglass brain teaser puzzles game from Novel Games.

In this game you are given a Brain Teaser to calculate the given time using the hourglasses. These hourglasses can measure 7 minutes and 11 minutes. Using these hourglasses you have to solve two problems. In one game your have to measure 15 minutes and in other game you have to measure 24 minutes. 
Lets see if you can solve this Hourglass Brain Teaser puzzle in minimum time.