Answer:
d. m = 15c
Explanation:
Directly proportional means as one amount increases, another amount increases at the same rate.
Proportionality can be used to set up an equation.
Given variables:
- m = amount of money earned (in dollars)
- c = number of cars washed
If m is directly proportional to c then:

Convert this to an equation by using a constant of proportionality, k:

We are told that Paul washed 8 cars and earned $120.
Substitute these values into the found equation and solve for k:



Finally, substitute the found constant of proportionality (k) into the equation:
