The direct variation of the time it takes to fill the tank with the volume of the tank may be presented by the following equation,
V = kt
where V is volume, t is time and k is the constant of direct variation.
To solve for k, substitute the known data in the equation above,
400 gal = k(1 hour) ; k = 400 gal/ h
For the second condition,
600 gal = (400 gal/h) t ; t = 1.5 hours
Therefore, it will take 1.5 hours to fill a 600-L tank.