In a theatre, the number of seats in successive rows forms an arithmetic progression. The 4th row contains 30 seats while the 12th row contains 70 seats. Answer the following:
(i) Find the number of seats in the first row and the increase in seats per row.
(ii) Determine the total number of seats in the first 20 rows.
(iii) Identify the row that contains exactly 90 seats.