This is a question on geometry. We are told that the said silo is a combination of two shapes, a cylinder and a hemisphere. To find the area of such a shape, we will simply find the areas of the individual shapes and add them up.
This is illustrated below.
The above illustrates a model of the silos with radius 20ft and height 40ft.
The area of the cylinder's surface is:

where: r = 20ft and h = 40 ft

The area of the hemisphere is:
