To find the number of apartments booked that produces the maximum revenue, we can determine the vertex of the quadratic function R(x) = 400x - 0.4x^2. The x-coordinate of the vertex represents the number of apartments that will maximize the revenue.
The vertex of a quadratic function in the form ax^2 + bx + c can be found using the formula x = -b / (2a).
In this case, a = -0.4, b = 400, and c = 0.
Using the formula, we can calculate the x-coordinate of the vertex:
x = -b / (2a)
= -(400) / (2 * (-0.4))
= -400 / (-0.8)
= 500
Therefore, the number of apartments booked that produces the maximum revenue is 500.