The number of units sold in  months is .

(a)

Find the average number of units of the  during first year .

The average number of units is .

Therefore the average number of units is .

(b)

The function  is continuous over closed interval , and differentiable on the open interval .

Therefore mean value theorem can be applied.

So, there exists at least one number  in  such that .

Apply derivative with respect to .

So .

Since function is in months, round the value to greatest integer.

Therefore from the Mean Value Theorem in the april,  equals to the average velocity.

(a) The average number of units is .

(b) April.