A telephone company in a town has $ 500$ subscribers on its list and collects fixed charges of $Rs$ $300/-$ per subscriber per year. The company proposes to increase the annual subscription and it is believed that for every increase of $Re$ $1/-$ one subscriber will discontinue the service. Find what increase will bring maximum profit?
My work
I assumed the amount increased per individual by $ y $
$ x $ be the subscriber left .
$$f(x)=(500-x)(300+y)$$
But I am not able to get relation in $x$ and $y$