Answer and Step-by-step explanation:
Dog meets the train in = 4 minutes
Train catches up the dog in = 12 minutes
Let x be the time when train leave the route,
Train catch up the dog in 12 minutes, the endpoint of route is x and the dog meet up the train in 4min, so the equation becomes:
            12 - x = 4
        To find out the value of x, subtract 12 from both sides, we get
            x = 4 – 12
             -x = -8
Which gives x = 8, when train leaves the endpoint of route.
