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.
Green Like a Leaf
Riddle:
I usually go with a special beef, I'm a green vegetable, like a leaf. I'm a yummy addition to your St. Patrick's Day dinner. If you eat me a lot, you might even get slimmer. What am I?