A convex lens is held `45 cm` above the bottom of an empty tank. The image of a point at the bottom of a tank is formed `36 cm` above the lens. Now, a liquid is poured into the tank to a depth of `40 cm.` It is found that the distance of the image of the same point on the bottom of the tank is `48 cm` above the lens. Find the refractive index of the liquid.