The power of a lens (biconvex) is 1 . 25 m ^ - 1 in particular...

Using Len's maker formula,

$P = \dfrac{1}{f} =$\left({\mu}_{1} - {\mu}_{2}\right)$\left(\dfrac{1}{{R}_{1}} - \dfrac{1}{{R}_{2}}\right)$

Where, ${\mu}_{1}$ is refractive index of lens and ${\mu}_{2}$ is of surrounding medium.

Putting the values,

$⇒ 1 . 25 = \left(1 . 5 - {\mu}_{2}\right)\left(\dfrac{1}{0 . 2} + \dfrac{1}{0 . 4}\right)$

$⇒ \dfrac{1 . 25 \times 0 . 08}{0 . 6} = \left(1 . 5 - {\mu}_{2}\right)$

The refractive index of surrounding medium is ${\mu}_{2} = \dfrac{4}{3}$.