Question

Rearrange the formula S = a/(a - r) for r.

Answer 1

S = a / (a - r)...multiply both sides by (a - r)
S(a - r) = a
Sa - Sr = a
Sa - a = Sr
(Sa - a) / S = r or it could be : (aS - a) / S = r

Answer 2

S = a/(a - r)
s(a - r) = a
sa - sr = a
sr = sa - a
sr = a(s - 1)
r = (a(s - 1))/s