Question 952152
Up=6mi/10mi/hr=0.6 hr
Down=6mi/15 mi/hr=0.4 hr
Distance=distance up+distance down=6mi+6mi=12mi
Total time=time up+time down=0.6hr+0.4hr=1 hour ANSWER 1: They were riding for 1 hour.
Average speed=total distance/total time=12 miles/1 hour=12 miles/hour 
ANSWER 2: Their average speed was 12 miles/hour.