If 7th and 13th terms of an A.P. be 34 and 64 respectively, then its 18th term is

Answer: Option C

7^th term (a₇) = a + 6d = 34
13^th term (a₁₃) = a + 12d = 64
Subtracting, 6d = 30 ⇒ d = 5
and a + 12 x 5 = 64
⇒ a + 60 = 64
⇒ a = 64 – 60 = 4
18^th term (a₁₈)
= a + 17d
= 4 + 17 x 5
= 4 + 85
= 89