the bottom of the pool is 15 x 30 = 450 sq feet.
 you the have 2 walls that are 30 x 5 = 150 x 2 = 300 square feet
 then 2 walls that are 15 x 5 = 75 x 2 = 150 square feet
 for a total of 450 +300 +150 = 900 square feet
 1 can covers 250 sq ft
so 900/250 = 3.6
 so you would need 4 cans of paint