Marco and his dad went strawberry picking. Marco's strawberries weighed 13 pounds. If together their strawberries weighed 21 pounds. How much did his dad's strawberries weigh?
1. Marco's strawberries weigh 13 pounds. 
2. Together, Marco and his dad's strawberries weigh 21 pounds. 
3. Therefore, his dad's strawberries weigh 8 pounds.