A company estimates that it will sell $N(x)$ units of a product after spending $x$ thousand dollars on advertising, as given by $$N(x) = -3 x^3 + 260 x^2 - 3200 x + 19000, \qquad 10 \leq x \leq 40.$$
(A) Use interval notation to indicate when the rate of change of sales $N'(x)$ is increasing.

Note: When using interval notation in WeBWorK, remember that: You use 'I' for $\infty$ and '-I' for $-\infty$, and 'U' for the union symbol.
If you have extra boxes, fill each in with an 'x'.

$N'(x)$ increasing:

(B) Use interval notation to indicate when the rate of change of sales $N'(x)$ is decreasing.

$N'(x)$ decreasing:

(C) Find the average of the $x$ values of all inflection points of $N(x)$.
Note: If there are no inflection points, enter -1000.

Average of inflection points =

(D) Find the maximum rate of change of sales.

Maximum rate of change of sales =