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

Given first term (a) = 3

Common difference (d) = n₂-n₁ = 8-3 = 5

a_n = 78

⇒ a_n = a + (n-1)d

78 = 3 +(n-1) 5

75 = (n-1) 5

(n-1) = 15

n = 16

Therefore 78 is 16^th term of the series.