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