SSC CGL 24 Aug 2021 Shift 3 Paper

From 1 to 50 Numbers
Multiples of $2=2,4,6,8\cdots 50$
Sum of multiples of 2
$=2+4+6+8+\cdots +50$
$=2\left[1+2+3+4+\cdots +25\right]$
$=\frac{2\times 25\times \left(25+1\right)}{2}$
$\left\{\because \frac{\mathit{n}\left(\mathit{n}+1\right)}{2}=1+2+\cdots +\mathit{n}\right\}$
$=25\times 26=650$
Multiples of $5=5,10,15,20,25,30$,
$35,40,45,50$
But, 10, 20, 30, 40 and 50 are already taken as multiples of 2 .
So, Remaining multiples of $5=5,15$,
$25,35,45$
$\therefore$ Sum of multiples of 5
$=5+15+25+35+45=125$
$\therefore$ Required average
$=\frac{650+125}{25+5}=\frac{775}{30}=25.8$