There are a fews ways to solve this. I am going to use the distance equation
d = rt.
d = distance = 600
r = speed = ?????
t = time = hours = 2.5
Lets find the rate/speed by plugging in what we know.
d = rt
600 = r2.5
600 /2.5 = r2.5 / 2.5
240 = r
r = 240
Now we got the rate/speed we can solve our problem. It wants to kow how long it will take to travel 1500 more miles from the 600 miles that has already traveled
d = rt
d = 1500
r = 240
t = ?????
1500 = 240t
1500 / 240 = 240t / 240
6.25 = t
t = 6.25.
It will take 6.25 or as fraction 6(1/4) hours more.