Question

The first term of an A.P is $5$ the common difference is $3$ and the last term is $80$; find the number of terms ?

Answer 1

$T_n=a+(n-1)d$
Where $a$ is the first term, $d$ is the common difference and $T_n$ denotes the n-th term.
Substituting accordingly from the question:
$80=5+(n-1)\cdot 3⟹n-1=25⟹n=26$