The problem states the following:
A container in the shape of a right circular cone with vertex angle a right angle is partially filled with water.
A. Suppose water is added at a rate of 3 cu.cm/sec. How fast is the water level rising when the height h=2cm?
B. Suppose instead no water is added, but water is being lost by evaporation. Show that the level falls at a constant rate. (You will have to make a reasonable physical assumption about the rate of water loss - state it clearly.)
I understand how to do part A, however, in the solution for part B, it is stated that one should assume that "the rate of evaporation is proportional to the surface area." I don't understand how they came to that conclusion. The full solution goes as follows:
