Marco and his dad went strawberry picking. Marco's strawberries weighed 21 pounds. If together their strawberries weighed 48 pounds. How much did his dad's strawberries weigh?
1. Marco's strawberries weigh 21 pounds. 
2. Together, Marco and his dad's strawberries weigh 48 pounds. 
3. Therefore, his dad's strawberries weigh 27 pounds.