Ian contributes $2000 a month into an account earning interest at a rate of r/year compounded monthly. At the end of 25 years his account will be worth

$\displaystyle S=((24000[(1+(r/12))^(300)-1])/(r))$

Find the differential of S.

I'm having trouble calculating the derivative of this equation because there are so many mixed operations. I know the differential is S^(prime)(r)(dy/dx)x but how can I find the differential if I can't find the derivative?