Question 364054
Solution :I= E/R+r for  R
subtract r on both sides
I-r =E/R + r - r
I -r = E/R
Multiply both sides by R
R*(I-r)= E/R *R
R*(I-r) = E
Divide both sides by (I-r)
R*(I-r)/(I-r) = E/(I-r)
R = E/(I-r)

Note : The basic idea here is to isolate R