[Ether]

Just use the two-point formula:

y = y1+(x-x1)/(x2-x1)*(y2-y1)

with x1=8, x2=13, y1=0, y2=100 this gives y = 20*(x-8)

or plot your data in Excel

8,0
9,20
12.5,90
13,100

to see the shape, and then

click on "add trendline" and select an appropriate model (linear in this case) to fit (and it will give you the equation)