During a 31 day (month) long Australasian advertising campaign the total sales of a particular magazine is given by the formula S(t)= 4t^2 +80t +12000.

a) Find the average of change in sales S

(i) from t=6 to t=13 days

(ii) from t=26 to t = 31 days

(b)Use differentiation to find the instantaneous rate of change of sales whent= 26 days.

(c) Interpret the result from (a) (ii) and (b) with respect sales expectation