Help. 15 ​

If you think about it, how many contributors she has already lined up is not necessary. They just put that in to throw you off. 

If each contributor is going to pay an equal amount. Then the cost/# contributors. 

\( C = {500 \over n}\)

So we simply do 500/20 to see how many new contributors Jane must recruit. 

C = 500/20 

C = 25 contributers. 

Julius got the correct answer....but a little clarification:

Cost per contributor = C = $ 500/n     where n= number of contributors

We want C=20

so    $ 20 = 500/n   now solve for n

      n =$ 500/$20

       n= 25  = number of contributors