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

Let $n$ be the number of terms. Here $a = 3$ and $d = 8 - 3 = 5$. We are given $a_n = 78$. Using the formula $a_n = a + (n-1)d$, we have $3 + (n-1)5 = 78$, which simplifies to $5(n-1) = 75$, so $n-1 = 15$, giving $n = 16$. Thus, the 16th term is 78.