13.5 would be half way between 13 and 14 on the number line. When you use 0.5 (1/2 or one half), the "half" is half way between two numbers, in this case, between 13 and 14.

13....13.5....14....14.5....15.... etc. I bolded the 13.5 so you would see if you make the dots

into a solid line, then 13.5 lies half way between 13 and 14. I included 4 dots between each number so it appears to be equal distance from the other numbers (as it should be).