I=V/R+r r is the subject

Question

Can anybody help solve the following equasion  (TRANSPOSE) I=V/R+r =????

make r The subject

Any help would be greatfully aprisiated.

Answer 1

 I=V/(R+r)

Switch I and R+r,

R+r = V/I

Add -R to both sides,

r = (V/I) - R <==Answer