Question 261454
You travel 2 hour at 35 mph, 3 hours at 55 mph and 4 hours at 70 mph.
Total distance travelled = 2*35+3*55+4*70
                         = 70+165+280
                         = 515 miles
Total time = 2+3+4 = 9 hours
Average speed = 515/9 = 57 2/9 miles/hour