Given, the refractive index of the material of the lens is μ1=1.5, the refractive index of the medium is μm=1.6 and the focal length of the lens in air is fa=20cm.
In air:
fa1=(μ1−1)[R2]
In liquid:
fm1=(μmμ1−1)[R2]
The focal length as measured in air and that as measured inside the liquid medium are related by,
fafm=(μ1−μm)(μ1−1)μm...(1)
Equation (1) implies that
20fm=(1.5−1.6)(1.5−1)1.6⇒fm=−160cm