Using Len's maker formula,

$P=\frac{1}{f}=\left({\mu }_1-{\mu }_2\right)\left(\frac{1}{R_1}-\frac{1}{R_2}\right)$ 

Where, ${\mu }_1$ is refractive index of lens and ${\mu }_2$ is of surrounding medium. 

Putting the values, 

$\Rightarrow 1.25=\left(1.5-{\mu }_2\right)\left(\frac{1}{0.2}+\frac{1}{0.4}\right)$

$\Rightarrow \frac{1.25\times 0.08}{0.6}=\left(1.5-{\mu }_2\right)$

The refractive index of surrounding medium is ${\mu }_2=\frac{4}{3}$.