Which term of the AP :

Solution:

$a=3, d=5$

Let $\mathrm{t}_{\mathrm{n}}=78$

$\Rightarrow a+(n-1) d=78$

$\Rightarrow 3+(n-1) \times 5=78 \Rightarrow 5 n-2=78$

$\Rightarrow 5 \mathrm{n}=80 \quad \Rightarrow \mathrm{n}=16$

Hence, $\mathrm{t}_{16}=78$