You have two set of points: (30, 85) and (45,77.5).
A line. You can get the equation of the line as (for instance):
y - y1 = m*(x-x1),
where (x1,y1) is any of your two points and m is the slope:
m = (y2-y1)/(x2-x1) = (77.5-85)/(45-30) = -1/2
y -85 = -0.5*(x-30) ===> y = 85 -0.5*x +15 = 100 - 0.5*x
Let's check:
for x= 45, y = 100-45*0.5 = 100- 22.5 = 77.5 and
for x= 30, y = 100 - 30*0.5 = 100-15=85
Yeah! :)