Question

solve A= P+Prt for r. Assume all variables represent positive values.

Answer 1

A = P + Prt
A = P/P + Prt/P
A = P(1 + rt)
A/P = P(1 + rt)/P
A/P = 1 + rt
A/P - 1 = 1 - 1 + rt
A/P - 1 = rt
(A/P - 1)/t = rt/t
(A/P - 1)/t = r

Thus r = (A/P - 1)/t.