Question 380321
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Hi,
Let x and (x+15mph) represent the speeds of the cars. x being the slower car's speed.
D = r*t OR D/r = t  Times are the same in this example, sets up the equality:
 120mi/x = 180mi/(x+15mph)
cross multiply to solve for x
120(x+15) = 180x
120*15 = 60x
30mph = x , the speed of the the slower car.  
the faster speed of the other car is 45mph    (30mph + 15 mph)
CHECKING our answer
  120mi/30mph = 180mi/45mph = 4hrs (misc info as to the time each traveled)