A motor-bike priced at Rs 68,000 is to be paid for through 20 monthly instalments that increase by a constant amount every month. The first instalment is Rs 1,500 and the last instalment is Rs 5,400.
(i) Justify that the series of instalments forms an arithmetic progression and determine its common difference.
(ii) Find the 12th instalment.
(iii) Calculate the total amount paid through all the instalments. State clearly whether the buyer pays more or less than the marked price of the bike and by how much.