A thin lens of refractive index 1.5, and focal length in air 20 cm is placed inside, a large container containing two immiscible liquids as shown below. If an object is placed at an infinite distance close to principle axis (common intersection line of layers), the distance between images

For upper portion, $f_1=\frac{{\mu }_2-1}{\frac{{\mu }_2}{{\mu }_1}-1}\left(f_a\right)=\frac{1.5-1}{\frac{1.5}{1.2}-1}\times 20=40cm$   $f_{a }=focal length of the lens in air.$$f_1 is the position of the image for the upper part of the object and f_2 is the position of the image for the lower part of the object.$

For lower portion, $f_1=\frac{{\mu }_2-1}{{\mu }_2/{\mu }_3-1}\left(f_a\right)=\frac{1.5-1}{1.5/2.5-1}\times 20=-25cm$ so the distance between the images is equal to 40+25=65 cm.