Question 437247
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Hi
 Four hours later train B leaves 
Let x and (x+4) represent time train B and A traveled respectively
Question states*** NOte: D = r*t, Distances same for both A and B
 20mph*(x+4) = 30mph*x
       80 = 10x
      x = 8hrs, time it takes train B to catch up to train A 

 CHECKING our Answer***
 20mph*12hr = 30mph*8hr = 240mi