Question

predict the changes in the focal length of a convex lensewhen it is kept water and kerosene

Answer 1

When a convex lens is kept in water, its focal length increases due to decrease in value of refractive index.
in Kerosene same thing happens. Focal length increases.

Answer 2

yes . This can be understood in the lens maker formula.

When convex lens is in air, it's focal length is given by

1/f (air)=(n-1)(1/R1)-(1/R2)..........(1)