Choose any two points on the line and find the rate of change between them.

We will the points at (10, 15) and (20, 20).

(10, 15)10$ sales, 15$ cost.

(20, 20)20$ sales, 20$ cost.

Rate of change = .

The cost changes from 20$ to 15$ and the sales changes from 20$ to 10$.

Rate of change =

Rate of change =

Rate of change = .               (Divide: )

So, the rate of change is , or a increase of 1$ per 2$ of sales.

The rate of change is .