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

Given A.P. is 3, 8, 13, 18,…
For the above AP,
a = 3
d = a2 − a1 = 8 − 3 = 5
Let nth term of this A.P. be 78.
an = a + (n − 1)d
⇒ 78 = 3 + (n − 1)5
⇒ 75 = (n − 1)5
⇒ (n − 1) = 15
∴ n = 16
Hence, 16th term of this A.P. is 78.