A real estate office handles 50 apartment unists. When the rent is $720 a month, all units are occupied. When the rent is increased by $40, one unit becomes vacant. Each occupied unit requires an average of $48 a month for maintenance. What rent should be charged to obtain max. profit.

My work:P(profit); C(cost); R(revenue); p(x)[price per unit];

so; and

and

And since I get x=16.6 I get a bit of doubt but then I test x=16 and x=17 on P(x) and get that to maximum profit x=17; so $1,400 should be charged.

Thanks for your time.