Question 408559
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Hi
Two trains leave a station at the same time. 
One travels east at 50mph and the other travels west at 55mph 
In how many hours will the two trains be 315 miles apart?
Let x and (315-x) represent the distance the 55mph and 50mph travel respectively

D = r*t OR t = D/r  (time for each train the same
 {{{ x/55 = (315-x)/50 }}}   |cross multiplying to solve for x 
 50x = 55(315-x)
 105x = 55*315
    x = 55*315/105 = 165mi, distance travel by 55mph train
  t = 165/55 = 3hr, number of hours it will take for trains to be 315 mi apart

CHECKING our Answer***
 3*55 + 3*50 = 165mi + 150mi = 315mi