The equation for a slope is rise/run. You know from the question that the slope is -4/5 in its simplest form. However, the run in its original form is 20.
So if the run is 5 in its simplest form, but 20 in its original form, then this means the slope must have been simplified by dividing both the original rise and the original run by 4, because 20/4 = 5.
That also means we can find out the original rise by doing the inverse of what we did above. We can multiply the rise in its simplest form by 4 to get the original. That gives us -16 (because -4*4 = -16).
