Let r be the speed of the faster car, in mph. Then the speed of the slower one is (r-15) mph. The "distance" equation is 4*r + 4*(r-15) = 420. 4r + 4r - 60 = 420, 8r = 420 + 60, 8r = 480 ---> r == 60. Answer. The speed of the faster car is 60 mph. That of the slower car is 60-15 = 45 mph.