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.
You have a 4 minute sand clock and a 7 minute sand clock, you need to measure 9 minutes while using both of them. What's your first move?
(The 'Google Riddles' are interview questions those who wish to get hired were asked).