Question 499953
d=rt is the basic distance equation
d = 877
t = d/r
.
r = 75 mph for 1/3 of trip
r = 70 mph for 1/3 of trip
r = 65 mph for 1/3 of trip
..
t = (877/3)/75 + (877/3)/70 + (877/3)/65
t = 12.5714042 hr
.
The average speed:  877 miles / 12.5714042 hr = 69.7614989 mph.
Note that it is close to 70 mph, but it is not = 70 mph.
.
Done.