Negative Number Line

Why does -2.5 - 0.5 equal - 3, but 2.5 - 0.5 equal 2?

Good question, it is important that you understand this :)

I am showing you on a number line.

When you are subtracting one numbers from another, you always start at the first number,  

and then move left towards the negative end of the numberline how ever many units you want to subtract.

So 

For      -2.5 - 0.5   start at -2.5 then go in the negative direction (left) 0.5 units and you will come to -3

For     2.5 - 0.5  start at 2.5 then go in the negative direction (left) 0.5 units and you will come to +2