Question 338967
let one car's speed be x
the other car's speed = x+10 
..
time taken by faster car = 3 hours
time taken by slower car = 3.5 hours
..
distance = speed * time
the distance they travel is the same
3(x+10)=3.5x
3x+30=3.5x
3.5x-3x=30
0.5x=30
x=30/0.5
x=60 mph. speed of one car.
the other car's speed = x+10 = 70 mph
distance traveled = speed * time 
= 70*3=210 miles