solve formula for the specified letter I=E/R+r for r

Question

help me solve this formula please.

Answer 1

I=E/(R+r)

multiply both sides by (R+r)

(R+r)(I)=(R+r)(E)/(R+r)

(R+r) cancels on the right side

(R+r)(I)=(E)

divide both sides by I

(R+r)(I)/I=E/I

I cancels on left side

R+r=E/I

subtract R from both sides

R+r-R+(E/I)-R

solution

r=(E/I)-R