Mary wants to bake 10 apple pies for a charity event. Each pie needs 8 apples and she already harvested 50 apples from the trees in her garden. How many more apples does she need to buy to make all 10 pies? Think carefully first, then make a decision:
She needs 10 * 8 = 80 apples to make the pies. So she has to buy 80 - 50 = 30 more apples. So, the answer is 30.