Betty's grade in science class is affected by the number of missing assignments. The table below shows the science grade percentage for a given number of missing assignments.
Number of missing assignments: 3,5,7,9
Science grade%: 80,70,60,50
The equation for the given table is y=-5x+95. If Betty is missing 15 assignments, what's the [percentage of her grade in science class?

What is the rate of change?