Question

If a+b=140 and ar+br=35. What is the value of r?

Answer 1

First this you need to do is solve for r
ar+br=35
r(a+b)=35
r=35/(a+b)

Answer 2

The next thing is use substitution so you only have one variable
a+b=140
a=140-b

Answer 3

I didn't think of turning r into terms of ab. Thanks!

Answer 4

Then substitute 140-b in the equation you solved for r
r=35/((140-b)+b)
r=35/140