Volume of a sphere with radius of 70 feet is:
[tex]V = \frac{4}{3} * \pi * r^{3} [/tex]
where r is that radius
Because we need quarter sized tank of that radius we need to divide formula for volume by 4
Than we get:
[tex]V = \frac{1}{3} * \pi * r^{3} [/tex]
V = 359188,76
Rounded:
V = 359189
