Which term of the AP : 3, 8, 13, 18, . . . ,is 78?

Here, a = 3 and d = 8 – 3 = 5.
Let, nth term of the A.P. is 78.
Therefore, a_n = 78
⇒ a + (n -1)d = 78
⇒ 3 + (n -1)(5) = 78
⇒ (n – 1)(5) = 75
⇒ n – 1 = 15
⇒ n = 16
Hence, 16th term of the A.P .: 3, 8, 13, 18, … is 78.