30 minutes is HALF an hour, so if you can run 5.5 miles in a WHOLE hour,
then you can run HALF that in a HALF hour.  HALF of 5.5 is the same as 
dividing 5.5 by 2: 

                       2.75  
                     2)5.50
                       4
                       1 5
                       1 4
                         10
                         10
                          0

Answer: 2.75 miles

Edwin