Matthew currently has enough money to buy 45 toy cars. if the cost of each car was 10 cents less, matthew could buy 5 more cars. how much money does matthew have to spend on toy cars?

The answer is $45

Let's make the system of equations.
x - the cost of the car
y - the amount Matthew has

Matthew currently has enough money to buy 45 toy cars:
y =45x
If the cost of each car was 10 cents less, Matthew could buy 5 more cars: 
y = (x - 10)*(45 + 5)

45x = (x - 0.10) * 50
45x = 50x - 5
5 = 50x - 45x
5 = 5x
x = 5 / 5
x = 1

y = 45x = 45 * 1 = $45