Question 1176879
Let the avg speed of Joe is x km/hr then avg speed of Bob = x+80 km/hr
 and the d is the distance of race
Bob had completed the race in 30 min
Thus d=(x+80)/2..............(1)
Distance covered by Joe in 30 min 
= x/2 =4/7*d
x/2= 4/7* (x+80)/2
x/2=2(x+80)/7
7x=4(x+80)
7x=4x+320
3x=320
x=320/3

Thus Joe's speed =106.66 km/hr

From(1)
d=(106.66+80)/2
=186.66/2=93.33 km

Thus Distance of race= 93.33 Km