hmm..
Unless I'm missing somethere here, you can't solve for N, simply because it cancels out once you can do anything else with the equation.
(S+.02+(C/N))-(.02+(C/N))=S
S + .02 +C/N - .02 -C/N=S
S+C/N-C/N+.02-.02=S
S+0+0=S
S=S
You see why? So N could be any real number, excluding zero, and the equality would still hold true.