Atelephone company in a town has 500 subscribers on its list and collects fixed changes of RS.300/-per subsriber per year. The company proposes to increase the annual subscription and it is believed that for every increase of Re.1/- on subscriber will discontinue the service. Find what increase will bring maximum profit..?/

If the increase be Rs. x per subscriber then the rate will be 300 + x and subscribers left will be 500-x 

therefore, acc. to the given condition. 

If I be the new income, then

I = (300+x) (500-x) 

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