Since the beginning of the year, weekly sales of a luxury good are found to have decreased exponentially. After t weeks, sales can be modelled by 3000e−0.02t. 

(a) Work out the weekly sales when t = 12 and t = 13 and hence find the decrease in sales during this time.
(b) Use differentiation to work out the rate of decrease in sales after 12 weeks and compare this with your answer to part (a).