You are told that this is a cone- if you do not know the formula for a cone and your textbook does not have it, you could "google" "volume of cone" and get . The surface of the water is a circle- it has area .
Also you suddenly introduce the formula as "rate of evaporation". Where did you get that? The left side is kinetic energy, the right side is potential energy under gravity. Evaporation, and this problem in general, has nothing to do with energy or gravity.
You are told that the water evaporates at a rate "proportional to the surface area". The surface area is the area of that top circle, . The water evaporating reduces the volume so that says the where "k" is the proportion.
Because that equation becomes .
Now, that involves two dependent variables, r and h. We need a formula connecting the two so we can replace one. That comes from "A conical tank of height 3m is full of water and the radius of surface is 1m." The height is 3 times the radius and as the water level decreases that ratio stays the same: h= 3r so we have .
You should be able to solve that for r, as a function of time, then find h= 3r, and so find V as a function.
pls i will greatly appreciate any idea on this, thanks