We have a direct variation. Distance, D, varies directly as time, t. 
So D = Kt where K is the constant. In this case, speed. So D = Kt is our constant of variation. 
If D = 150 miles and t = 4 hours 
Then we have 150 = K (4) 
K = 150/4 = 37.5. The constant of variation is 37.5. 
If you keep traveling at a constant speed K = 37.5 and D = 200. We have 
200 = 37.5 * t 
t = 200/37.5 = 5.333 hours.