Question 286487
We will make use of the formula D=VT, where D=distance, V=velocity (speed) and T=time

Let Dn,Vn = Distance travelled by the north car, velocity of the north car
    Ds, Vs = Distance travelled by the south car, velocity of the south car

The times they travel are the same, because they left at the same time. So tn=ts=t

We know that the two distances, Dn and Ds, add up to 540 miles:

Dn + Ds = 540

Now put in the formulas D=VT, instead of Dn and Ds:

Vn*t + Vs*t = 540
65t + 55t = 540
120t = 540
t = 4.5 hours

The cars will be 540 miles apart in 4.5 hours.