Two thin convex lenses are kept in contact coaxially. If the focal length of the combination of the lenses is 4 cm and sum of the focal lengths of the two lenses is 18 cm , then the focal length of the lens of low power is

If the far point of a short sighted person is 400 cm , then the power of the lens required to enable him to see very distant objects clearly is

When a convex lens is immersed in a liquid of refractive index equal to $80 \%$ of the refractive index of the material of the lens. The focal length of the lens increases by $100 \%$. The refractive index of the liquid is