First, calculate volume:
volume = 21.2 ft * 46.1 ft * 19.4 ft
volume = 18,960 ft^3
Convert to gallon:
volume = 18,960 ft^3 * (12 in / 1 ft)^3 * (1 gallon / 231 in^3)
volume = 141,830.71 gallon
Therefore the time is:
time = 141,830.71 gallon / (5.85 gal/min)
time = 24,244.57 min