Well you know that in order to find the rate you have to divide the distance by the time, right? Right! So you do: 30 divided by 1 which equals 30. Then you do: 120 divided by 4 which also equals 30. So then you already know that there is no rate of change just by looking at it.
But just to show you: 30 - 30 = 0
So there you go, no rate of change!
Hope this helps! =]