The Seven Candles
You have a 7-minute hourglass and an 11-minute hourglass. How do you measure exactly 15 minutes?
Answer: Start both hourglasses. When the 7-minute hourglass runs out, turn it over (7 minutes elapsed). When the 11-minute hourglass runs out, turn the 7-minute one over again (11 minutes elapsed, with 4 minutes left in it). When the 7-minute hourglass runs o…