$200 gm$ of an oleum sample (labelled as $109\%$) is mixed with $400 gm$ of another oleum sample (labelled as $118\%$). The labelling of the new sample formed will be: 

$200 \mathrm{gm} 109\% \to {\mathrm{SO}}_3\left(80\right)$

$200 \mathrm{gm} 109\% \to {\mathrm{H}}_2{\mathrm{SO}}_4\left(120\right)$

$400 \mathrm{gm} 118\% \to {\mathrm{SO}}_3(80\times 4=320 \mathrm{gm})$

$400 \mathrm{gm} 118\% \to {\mathrm{H}}_2{\mathrm{SO}}_4(80 \mathrm{gm})$

$600 \mathrm{gm} \to {\mathrm{SO}}_3\left(400\right)$

$600 \mathrm{gm} \to {\mathrm{H}}_2{\mathrm{SO}}_4\left(200\right)$

${\mathrm{SO}}_3+{\mathrm{H}}_2\mathrm{O}⟶400$

$55\times 18=90 \mathrm{gm}$

For $100 gm$ sample 

$=100+\frac{90}{6}$

$=115\mathrm{\%}$