(a) With a unit rate of 8, repeated (iterated) 72 times, we can simply multiply 8 by 72 to get the net change: 8 × 72 = 576 fish.
(b) For the unit rate (i.e. rate of change per time - minutes in this case) we can simply divide the total water pumped out by the total time it took to perform this. So: 48 ÷ 8 = 6 gallons/min.
Hope I could help you! :)