Marco and his dad went strawberry picking. Together they collected strawberries that weighed 24 pounds. On the way back Marco lost 9 pounds of strawberries. Marco's strawberries now weighed 3 pounds. How much did his dad's strawberries weigh?
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First, we know that Marco and his dad went strawberry picking and together they collected strawberries that weighed 24 pounds. 

Next, we know that on the way back Marco lost 9 pounds of strawberries. 

That means that Marco's strawberries now weigh 3 pounds. 

Now we just have to solve for his dad's strawberries. 

We know that the total weight of the strawberries is 24 pounds. 

We also know that Marco's strawberries weigh 3 pounds. 

That means that his dad's strawberries weigh 21 pounds.