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.
Party Animal
Riddle:
I often join you at parties and holidays, but I never get to enjoy them.
What am I?
I have around 5,000 feathers and I'm a bird native to America. People say Benjamin Franklin wanted me to be the national bird of the United States.
What am I?