$\rightarrow$ An Ideal solution is prepared by mixing $1\, mol$ $A$ and $2 \,mol\, B$.

$\rightarrow X_{A}=\frac{1}{3}$ and $X_{B}=\frac{2}{3}$

$\rightarrow$ Acc to Raoult's low

$P _{ T }= X _{ A } P _{ A }^{0}+ X _{ B } P _{ B }^{0}$

$P _{ T }=\left(\frac{1}{3} \times 21\right)+\left(\frac{2}{3} \times 18\right)$

$P _{ T }=7+12=19\, KPa$