The yearly profit (in thousand rupees) of a stationery company from selling n thousand notebooks is modelled by the polynomial
P(n)=−2n2+50n−200.
A rival company’s yearly profit (in thousand rupees) from the same market is given by
Q(n)=−n2+40n−150.
(i) Find the polynomial D(n)=P(n)−Q(n) representing how much more profit (in thousand rupees) the stationery company makes than its rival.
(ii) Calculate this profit difference when n=20.
(iii) Interpret the sign of each coefficient in D(n) with respect to how the difference in profits changes as the number of notebooks sold increases.