You have two hour glasses: A seven minute one and an 11 minute one. Using just these two hour glasses, accurately time 15 minutes.
You can try
1. Start both hourglasses.
2. When the seven minutes runs out turn it.
3. When the eleven minutes runs out, turn the seven minute hour glass again. It now has 4 minutes of sand.
4. When the seven minutes hourglass runs out, you will have 15 minutes.
The method is dependant on the accuracy of the hour glasses and your speed at turning them. Not 100% accurate.
Start the 11 minute hourglass and the 7 minute hourglass at the same time, as soon as the 7 minute one runs out start timing, once it fully runs down run it again for the full time you would be 15 minutes by the end of the timing