Help with Excel
 

I want to be able to calculate change gears for a gear hobber and was hoping to use excel `to do this.
I need to solve
a/b*c/d=known ratio
a,b,c,d
are the available change gears from 20 to 120 (number of teeth on the gear)
can someone please help?
cheers andrew

How is this related to building a MechMate?

a/b*c/d=known ratio

Formula for your cell will be

=sum(a/b)*(c/d)

but I do not know how to make is pretty where you can just plug in the numbers