The computer in my car displays the fuel consumption in
  miles per gallon (mpg) averaged over the distance travelled
  since the 'cancel' button was last pressed. At the start of
  today's 60 mile journey it read 36.0 mpg, and I failed to
  cancel this reading. After travelling the first 20 miles of
  today's journey the reading was 34.0 mpg, and after a further
  20 miles it was 33.0 mpg. Assuming that the fuel consumed per
  mile remained constant during today's journey, calculate:

  1. The distance the car had travelled since the last press of
  the cancel button before the start of today's journey,

  2. The actual mpg during today's journey,

  3. The mpg reading at the end of today's journey.