The surface area of the silo's side is 20x147 = 2940 square feet

Then add the area of the top, if flat, is pi x radius squared = pi(r)^{2}

The circumference is pi x the diameter or 2pi(r) = 147

r= 147/2pi

top area = pi(r)^{2} = pi(147/2pi)^{2} = 147^{2}/4pi = 3439.18

total area = sides + top = 2940 + 3439.18 = 6379.18 square feet

divide that by 300 to get the number of gallons needed

6379.18/300 = 21.2639 gallons

a little more than 21 1/4 gallons