Question

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

Answer 1

$3, 8, 13, 18, ….$
For this A.P., $a = 3$ and $d = a_2 − a_1 = 8 − 3 = 5$
Let nth term of this A.P. be 78.
$a_n = a + (n − 1) d$
$78 = 3 + (n − 1) 5$
$75 = (n − 1) 5$
$(n − 1) = 15$
$n = 16$

Hence, $16^{th}$ term of this A.P. is 78.