Question 1071439
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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 = {{{480/8}}} = 60.


<U>Answer</U>.  The speed of the faster car is 60 mph. That of the slower car is 60-15 = 45 mph.
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Solved.