Question 18999
Ohm's law for the amount of current (I) in a cicuit having (R+r) ohms of resistance with an applied voltage of E volts.

{{{I = E/(R+r)}}} Solve for R, multiply both sides by (R+r).
{{{I(R+r) = E}}} Expand (distributive property) the left side.
{{{IR + Ir = E}}} Subtract Ir from both sides.
{{{IR = E - Ir}}} Finally divide both sides by I.
{{{R = (E - Ir)/I}}}