# Derivation of formula for finding median for grouped data

I know the formula of formula for finding median for grouped data that is $$Median = L_m + \left [ \frac { \frac{n}{2} - F_{m-1} }{f_m} \right ] \times c$$ and I know what all the letters stand for. But can anyone provide a derivation of this. Because I am very curious on how this comes.

-