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 

Answer by Alan3354(69443)   (Show Source): You can put this solution on YOUR website!
 In 4 hours, train A is 80 miles away (20*4)

Train B "gains on" train A at 10 mph (30 - 20)

80 mi/10 mph = 8 hours.

 

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