An internet service provider in a town has 500 subscribers and charging ₹300 per month from each subscriber. He proposes to increase the monthly charges and it is believed from past experience that for every increase of ₹1, one subscriber will discontinue the service.

If ₹x is the monthly increase in subscription amount, then the number of subscribers are:

500-x

Total subscribers = 500

Increase of ₹x will decrease the subscribers by x 

∴ 500 - x