The time it takes to fill a tank varies directly with the volume of the tank. if it takes 1 hour to fill a 400 gallone tank, how long will it take to fill a 600 gallon tank.

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.