Make S the subject for;

F - S
D = ----- x 100
S

I assume you are starting with this:


D = (\frac {F-S} S ) \times 100.


Multiply both sides by S, and rearrange:


DS = (F-S)100\\
DS + 100S = 100F\\
S(D+100) = 100F\\
S = \frac {100F}{D + 100}