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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