You have two hourglasses, a 7 minute one and an 11 minute one. Using just these two hourglasses, how can you accurately time 15 minutes?
Answer
Start both hourglasses at the same time.
When the 7-minute hourglass runs out, turn it over immediately. At this time, the 11-minute hourglass will have 4 minutes remaining.
When the 11-minute hourglass runs out, turn over the 7-minute hourglass immediately. The 7-minute hourglass would have run for 4 minutes.
When the 7-minute hourglass runs out, 15 minutes would have elapsed.