SSC CHSL 6 Dec 2015 Shift 1 Paper

Odd integers between 2 and 22 = 3, 5, 7,......, 21
This is an A.P. with first term = a = 3 d = 2
Let number of terms = n
⇒ Last term = $\mathit{a}+\left(\mathit{n}-1\right)\mathit{d}$
⇒ $3+\left(\mathit{n}-1\right)2=21$
⇒ $\left(\mathit{n}-1\right)2=21-3=18$
⇒ $\left(\mathit{n}-1\right)=\frac{18}{2}=9$
⇒$\mathit{n}=9+1=10$
Sum of series = $\frac{\mathit{n}}{2}\left(\mathit{a}+\mathit{l}\right)$
= $\frac{10}{2}\left(3+21\right)$
$5\times 24=120$
∴ Average = $\frac{120}{10}=12$