Question

Leah has sold magazine subscriptions worth $330 for a school fundraiser. If she reaches a total of $500, she wins a gift certificate. How many more subscriptions does she need to sell to reach $500?

Answer 1

You can solve by subtracting: 500-330

but I like to use mental math and think this way:
she has 330
70 more will give her 400
and 100 more will give her 500
100 + 70 is 170.