Here, First term = a = 9

Common difference = d = 13 - 9 = 4

To find = 20^th term, ∴ n = 20

Using the formula for finding n^th term of an A.P.,

a_n = a + (n - 1) × d

∴ a_n = 9 + (20 - 1) × 4

⇒ a_n = 9 + 19 × 4 = 9 + 76 = 85

∴ 20^th term of the given AP is 85.