I assume you meant to write R(x)=100x/(b+x).

a) This translates to "When x = 40, R = .5. Find R(x)". Which means "Use R(40)=.5 to find b, and then write down the formula for R(x) with that b."

b) This translates to "Using the R(x) found in part (a), find R(60).".

Part (a) amounts to solving an equation for b ("Use R(40)=.5 to find b"), and Part b is just plugging x=60 and the discovered value for b into the formula.

I hope that helps clarify how you should proceed.