Final answer:
The maximum rate of drainage in millions of gallons per day is 16 million gallons per day.
Step-by-step explanation:
To find the maximum rate of drainage in millions of gallons per day, we need to divide the total volume of water in the reservoir (400 million gallons) by the number of days it will take to drain the reservoir (25 days). This will give us the average rate of drainage per day.
So, the maximum rate of drainage in millions of gallons per day is:
400 million gallons ÷ 25 days = 16 million gallons per day