The problem at hand is on related rates, which could be solved using differential calculus
The volume of a sphere: V = (4/3)(pi)(r^3)
Differentiating with respect to time,
(dV/dt) = 4(pi)(r^2)(dr/dt)
Substituting the given,
(dV/dt) = 100 , r = 5
100 = 4(pi)(5^2)(dr/dt)
Solving for (dr/dt)
dr/dt= 0.318 ft/s