You have a ten liter jug, an eight liter jug, and a seven liter jug. Obtain exactly four liters of water in the 10 liter jug. You have an infinite supply of water.
1. Fill the 7 liter jug completely with water.
2. Pour the water from the 7 liter jug into the 10 liter jug.
3. Fill the 7 liter jug again.
4. Pour water from the 7 liter jug into the 10 liter jug until the 10 liter jug is full (which will leave 4 liters of water in the 7 liter jug).
5. You now have exactly 4 liters of water in the 10 liter jug.