An apple grower finds that an apple tree produces on an average 400 apples per years, if no more than 16 trees are planted in a limit area. For each additional tree planted per unit area, the grower finds that the yield decreases by 20 apples per tree per years. How many trees should the grower plant per unit area so as to get the maximum yield?

Please teach me how to form the equitions... don't just give me answer... Thanks