You turn the 7 minute one and the 11 minutes one at the same time. When the 7 one finishes, turn it again. When the 11 one finishes, you know that 4 minutes have passed, with 4 minutes to get to the desired 15, all you need to do now is turn the 7 minute hourglass again since it counted 4 minutes, and when it runs out it'll be exactly 15 minutes.
There are two planes. One is going from Los Angeles to Japan at a speed of 600 MPH. The other is traveling from Japan to Los Angeles at a speed of 500 MPH.
When the planes meet which one will be closer to Japan?