rounding special case

Hi guys, need a little help rounding numbers in a special case, and was wandering if i can get help from brilliant minds in the community

X.X number needs to be rounded down, to nearest .5 or rounded up depending on the decimal number.
for example.
.1, .2 gets rounded down to .0
.3 -.7 gets rounded to .5
.8, .9 gets rounded up.

i'm fiddling with mod and ceil / floor with no success.

thanks in advance.