Given,

$n_1=10, {\overline{x}}_1=30, C{V}_1=50$
$n_2=20, {\overline{x}}_1=25, C{V}_2=60$
$C{V}_1=\frac{{\sigma }_1}{{\overline{x}}_1}\times 100\Rightarrow 50=\frac{{\sigma }_1}{30}\times 100\Rightarrow {\sigma }_1=15$
$C{V}_2=\frac{{\sigma }_2}{{\overline{x}}_2}\times 100\Rightarrow 60=\frac{{\sigma }_1}{25}\times 100\Rightarrow {\sigma }_2=15$
Combined mean $=\frac{10\times 30+20\times 25}{30}=\frac{80}{3}$
$d_1={\overline{x}}_1-\overline{x}=\frac{10}{3}, d_2=\overline{x}-{\overline{x}}_2=\frac{5}{3}$
Combined variance $=\frac{n_1{\sigma }_1^2+n_2{\sigma }_2^2+n_1{d}_1^2+n_2{d}_2^2}{n_1+n_2}$
$=\frac{2250+4500+\frac{1000}{9}+\frac{500}{9}}{30}$
$=\frac{6225}{27}=\frac{2075}{9}$