You are asking two questions which really are virtually the same (24+30=54) and (23.5+30.5=54). This is true because the are really the same numbers (when being subtracted) in disguise. First, think of how you get the number 23.5, when you start with 24 (You have to subtract 1/2 or 0.5). Next, think of how get the number 30.5, when you start with 30 (You have to add 1/2 or 0.5). So, in the first instance (24 to 23.5) you have subtracted 0.5, and the second instance (30 to 30.5) you have added 0.5. The net total (0.5-0.5) is Zero. Therefore, as you can see its just like adding 24 and 30, it will always give you 50, likewise adding 23.5 and 30.5 will give you the same. On another note, when you mark these points on a number line-you will see that the distance between 23.5 and 30.5 is 7 units, and the distance between 24 and 30 is six units. This is simply because you are measuring an absolute distance between two points, and not subtracting them. Anytime you move left on a number graph(where positive numbers go to the right) you will be subtracting, but the number graph can only show displacement relative to a predetermined point, so even though there is greater displacement (distance between the two points) this does not mean the sum quantity has increased. Remember when performing a math operation like subtraction there is a net loss,and moving in the left direction on a numberline would represent a net loss or subtraction. Ex. If I have 23.5 apples, the half apple missing does not exist, but the number line will represent this 0.5 apple missing as a 0.5 displacement in the left direction. God I hope all this makes sense, because I am thoroughly confused now.