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) 

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