The Hourglass Puzzle-Brain teaser
Riddle
You have two hourglasses: one measures 7 minutes and the other measures 4 minutes. How can you measure exactly 9 minutes?
1. Start both hourglasses at the same time. 2. When the 4-minute hourglass runs out, flip it immediately. 3. When the 7-minute hourglass runs out, flip it immediately. 4. When the 4-minute hourglass runs out again (at 8 minutes), there is 1 minute of sand left in the 7-minute hourglass. 5. Let that 1 minute pass. You're now measured exactly 9 minutes.