Check your optics skills! Double refraction or trouble refraction

A long cylindrical tube containing water is closed by an equiconvex lens of focal length $20 \text{ cm}$ in air. A point source is placed along the axis of the tube outside it at a distance of $50 cm$ from the lens. Find the distance from the centre of the lens where the image is formed.

Additional data :- $n_\text{lens} = 1.5$ and $n_\text{water} = \frac{4}{3} \approx 1.33$

Note : In solving the question you may use the following approximation :- $0.67 \approx \frac{2}{3}$ .