Hi, 

Let x represent the amount of time Train B traveled.  

Then (x+7hr) represents the amount of  time train A traveled

distance = rate*time.  distance the same for both Train and Train B

60mph*(x+7hr) = 80mph*x

solving for x

60mph*x+ 60mph*7 = 80mph*x

60mph*7hr = 80mph*x -60mph*x+  

420mi = 20mph*x

21hrs = x

Takes Train B 21hrs to catch up with Train A

checking our answer

60mph*28hr = 80mph*21hr = 1680mi 

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