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

First term = a = 3,

Common difference = d = 8 – 3 = 13 – 8 = 5

\(a_n\) = 78

Using formula \(a_n = a + (n - 1)d\), to find \(n^{th}\) term of arithmetic progression,

\(\Rightarrow \) 78 = 3 + (n - 1) 5
\(\Rightarrow \) 75 = 5n - 5
\(\Rightarrow \) 80 = 5n
\(\Rightarrow \) n = 16

It means \(16^{th}\) term of the given AP is equal to 78.