Hopes this helps please correct me if I am wrong:
Answer: f = rn/ 1-r
1. Multiply both sides by n + f.
r(n + f) = f
2. Expand.
rn + r f= f
3. Subtract r f from both sides.
rn = f - r f
4. Factor out the common factor f.
rn = f(1 - r)
5. Divided by sides by 1 - r.
rn/ 1 - r = f
6. Switch sides.
f = rn/ 1 - r