The time t required to empty a wastewater-holding tank is inversely proportional to the cross-sectional area a of the drainage pipe. If it takes 5 hours to empty with an area of 25 m^2, how long will it take to empty with an area of 36 m^2?
a) 3.33 hours
b) 5 hours
c) 6.67 hours
d) 10 hours