Let the speed of the slower train = y

and the speed of the faster train = x

Given, x - y = 15 ...........1

Again, given the slower train covers in 4 hours what the faster one covers in 3 hours

=> 3x = 4y

=> x = 4y/3

From equation 1, we get

      (4y/3) - y = 15

=> (4y - 3y)/3 = 15

=> y/3 = 15

=> y = 3*15

=> y = 45 km/h

Now, x - 45 = 15

=> x = 45 + 15

=> x = 60 km/h

So, the speed of the slower train 45 km/h and the speed of the faster train 60 km/h