James has a collection of 20 miniature cars and trucks. Cars make up 40% of the collection. If James adds only cars to his collection, how many cars must he add to make the collection 75% cars?
There are 40%*20 = 8 cars. Let n be the number of card James adds. Then
(8 + n)/(20) = 0.75
8 + n = 15
n = 7
James must add 7 cars.