Revenue = (Number of chairs)(cost per chair)

Currently, $R = (30)(25) = 750$

As per the prediction, $R = (30 + x)(25 - x)$, where $x = 1, 2, 3, ......$

Now, $R_1 = (31)(24) = 744$

$R_2 = (32)(23) = 736$

$R_3 = (33)(22) = 726$

We can see the values are decreasing, so the Maximum revenue could be $750$ only


Alternative Approach:

Let's solve the quadratic, $R = (30 + x)(25 - x)$
$R = -x^2 - 5x + 750$

Parabola is a quadratic function with the maximum value at the vertex

x-coordinate of the vertex is given by $\frac{-b}{2a} = \frac{-(-5)}{(2)(-1)} = \frac{-5}{2}$

This means - Maximum revenue would be generated if the number of chairs are $27.5$ and the cost per chair is $27.5$ too, which is not possible!