Question about round function

Question about round function

Good Evening.

Maybe an easy question but I am trying to understand how the 'round' function is working.
So, I read the tutorial but my question is the following:

If I have a number lets say 13.26 how can I round it to 13.50?
Another examples:

13.26 -> 13.50
13.24 -> 13.00
13.76 -> 14.00
13.74 -> 13.50

I think that you get my example. I do not know how can I do it with the round function.