Question 780263
Let the speed of the faster train be x miles per hour.
Then, speed of the slower train is (x-15) miles per hour.


Distance travelled by the faster train in 6 hours is 6x miles.
Distance travelled by the slower train in 6 hours is 6(x-15) miles.


Total distance travelled: 
6x + 6(x-15) = 630
12x - 90 = 630
12x = 720
x = 60


So the speed of the faster train is 60 miles per hour and that of the slower train is 45 miles per hour.