A painter is paid according to an arithmetic scheme. He receives ₹18{,}000 for the 4th house he paints and ₹21{,}000 for the 10th house.
(i) Find the amount he received for the first house and the fixed increase in payment per house.
(ii) Determine how much he will receive for painting the 20th house.
(iii) Calculate the total amount he will earn for painting the first 20 houses.