Question 619487
Train 1: 


d = rt


d = 120t


Train 2:


d = rt


d = 130t


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So train 1 has traveled 120t miles in t hours and train 2 has traveled 130t miles in t hours.


Since they are going in opposite directions (along let's assume a straight track), they will be 120t+130t = 250t miles apart after t hours.


We want this distance to be 750, so 


250t = 750


t = 750/250


t = 3


So in <font color="red">3 hours</font>, the two trains will be 750 miles apart.