Add :
-1.25 + 2.5 = 1.25
For plotting the first one we first need to see the integer as here is 1 we will move to 1. Now, here it is given 0.25. Each line between 1 and 2 has 10 numbers gap so, as it 25 > 20 and 25 < 30. So, we should plot it between the 2nd and the 3rd line. As it is 5 it should be in between the line.
Refer the plotting for better identification
Add :
4.2 + (-0.4)
4.2 - 0.4
3.8
For plotting this one we will move to 3 than it has 8 < 10 so it should be plotted within the first dot.
Refer the attachment for better identification