The given expression is
[tex] D = \frac{C-S}{N} [/tex]
And we have to solve for N.
TO solve for N, first we do cross multiplication, that is
[tex] DN=C-S [/tex]
And to solve for N, we have to divide both sides by D, that is
[tex] N = \frac{C-S}{D} [/tex]
And that's the required value of N .