Solutions:

The A.P. series is given as 3, 8, 13, 18, …

The first term, a = 3

The common difference, d = a₂ − a₁ = 8 − 3 = 5

Let the n^th term of the given A.P. be 78. Now as we all know,

As a result,

78 = 3+(n −1)5

75 = (n−1)5

(n−1) = 15

n = 16

As a result, the 16^th term of this A.P. is 78.