I flop around on sticks and sometimes you cheer me as I do, I desperately need a white powder to do what needs to be done, and looking at me you might wonder why I look like I am about to go swimming. What am I?
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.