Acompany introduces a new product for which the number of units sold s is
s(t)= 200 ( 5- 9/(2+t))
where t is the time in months

(a) find the avg. value of s(t) during the first year.
(b) during what month does s'(t) equal the avg value during the first year?

(a) The "average value" of a function over an interval [a,b] is defined to be 

(1/(b-a)) times the integral of f from the limits x= a to x = b. 

Now S = 200(5 - 9/(2+t)) 

The average value of S during the first year (from t = 0 months to t = 12 months) is then: 

(1/12) times the integral of 200(5 - 9/(2+t)) from t = 0 to t = 12 

or 200/12 times the integral of (5 - 9/(2+t)) from t= 0 to t = 12 

This equals 200/12 * (5t -9ln(2+t)) 

Evaluating this with the limits t= 0 to t = 12 gives: 

708.113 units., which is the average value of S(t) during the first year. 

(b). We need to find S'(t), and then equate this with the average value. 

Now S'(t) = 1800/(t+2)^2 

So you're left with solving 1800/(t+2)^2 = 708.113 

I'll leave that to you