Using Len's maker formula,
P=f1=(μ1−μ2)(R11−R21)
Where, μ1 is refractive index of lens and μ2 is of surrounding medium.
Putting the values,
⇒1.25=(1.5−μ2)(0.21+0.41)
⇒0.61.25×0.08=(1.5−μ2)
The refractive index of surrounding medium is μ2=34.