posted by Kenneth .
Marlene rides her bycicle to her friend Jon's house and returns home by the same route. Marlene rides her bike at constant speeds of 6mph on level grond, 4mph when going uphill and 12mph when going downhill. If her total riding time was 2 hours, how far was John's house? (note: Marlene does travel uphill, downhill, and on even ground)