Two ways.  Without or with algebra.  Here are both ways:

Without algebra.

Their rate of separation is 180 miles per 1.5 hours and 180÷1.5 = 120 mph.
Ted's speed made up 65mph of that 120 mph and so Sue made up the rest of
that 120mph speed of separation or 120mph-65mph or 55 mph.

With algebra.

Let x = Sue's rate.

Make this chart with the rates and their times of 1.5 hrs each:

       Distance        Rate      Time
Ted                     65        1.5
Sue                      x        1.5
-------------------------------------
Total    180

Now fill in the distances using DISTANCE = RATE×TIME

       Distance        Rate      Time
Ted     65(1.5)         65        1.5
Sue     1.5x             x        1.5
-------------------------------------
Total    180

The equation comes from:

                    

                      65(1.5) + 1.5x = 180
                        97.5  + 1.5x = 180
                                1.5x = 82.5
                                   x = 
                                   x = 55 mph

So Sue was driving 55 mph.

Edwin