Average rate of change of volume = (change in volume) / (time for the change)
Change in volume = (591 in³ - 1,000 in³) = -409 in³
Time for the change = 18 minutes
Average rate of change of volume = (-409 in³) / (18 min) = -22.722... in³ per minute
Rounded to the nearest hundredth, that's -22.72 in³ per minute .