500
                                Write an equation and then solve. Mary had some change in her pocket. She found 3 dimes and a nickel on the sidewalk and now has 90 cents. How much change did Mary have to begin with? (Hint: Work backward to solve!)
                                c + (3 x 10) + (1 x 5) = 90; c = 55, so Mary began with 55 cents.