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**Soul to soul** A real estate office manages 50 apartments in a downtown building. When the rent is $900 per month, all the units are occupied. For every $25 increase in rent, one unit becomes vacant. On average, all units require $75 in maintenance and repairs each month. How much rent should the real estate office charge to maximize profits?

I did it like this: let x represents the number of increase or decrease in rent and apartment, then I got the equation:

Revenue(x)= [(900+25x)(50-x)]-(50*75)

but I know I must did something wrong... Can someone help me with this?