SOLUTION :
Vol. of mixture $\left( CO + H _{2}\right)=60 mL$
Vol. of $CO _{2}$ formed $=20 mL$
From reaction: $2 CO (g)+ O _{2}( g )=2 CO _{2}( g )$
2 vol. $\quad 1$ vol. 2 vol.
Vol. of $CO _{2}$ formed $=$ vol. of $CO$ reacted
$\therefore$ Vol. of $C O$ which reacted $=20 mL , \%$ of $C O$ in the mixture $=\frac{20}{60} \times 100=33.33 \%$.