In order to answer this question, we use the concept of ratio and proportion wherein we use the equation,
R1 / T2 = R2 / T1
where R is for rate and T is for time. Substituting the known values to the equation,
(600)/x = (400)/ 20
The value of x from the equation is 30. Thus, it will take 30 hours if the rate was adjusted to 400 gallons per minute.