After spending $30 on a dress, Mary had 3/8 of her money left. How much money did she have at first?

The answer is that she had 48. Set it up as an equation with x as the amount of money she started with. x - 30 = 3/8x. That says, in words, "the amount of money she started with minus 30 for the dress left her with 3/8 of the money she started with. Solve for x by adding 30 to both sides and subtracting 3/8 from 8/8x (8/8 = 1). 8/8 - 3/8 = 5/8, so 5/8x = 30, multiply both sides by 8 to get 5x=240 and x = 48.