5) A surface water source contains 5000 units / of pathogenic bacteria. According to the relevant local environmental standard, it is desired to reduce the number of bacteria to 100 units/L at the outlet of the collection tank. Since the inlet flow rate of water to the reservoir is 460m3/h, how many series-connected equalization tanks with a total volume of 355000m3 should be used? (=0.25 day 1)