How could you use a number line to assist in adding or subtracting rational numbers?

One example of number line of rational numbers is shown below

To help with addition, for example, 0.1 + 0.3, you'd start from 0.1 on the number line and then hop on three lines to the right. You should end at 0.4 and that's the answer

To help with subtraction, you would have to 'hop back'.