Question 432456
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Hi
Let x and (x+1)represent the times 90mph & 75mph train travel before they meet
Note: D = r*t and distance traveled the same by both trains
  90x = 75(x+1)
  15x = 75
    x = 5hr, time 90mph(passenger train) travels before they meet 

CHECKING our Answer***
  90mph*5hr = 75mph*6hr = 450mi