this is a straightforward problem,
circumfrence = 2 * pi * r
r = diameter /2 = 10 feet
the circumfrence of the water tower is 20 pi feet
imagine if you unrolled the tower it is a rectangle that is 40 feet high and 62.83 feet wide which equals 2513.2 sq feet
2513.2/900 = 2.792 gallons needed to paint the towers outside and double that to paint the inside as well