Eh... there are no such property as damage resistance of /5... :p
There are two properties: Resistance and Soaking
Resistance are written like 5/- and only versus a specific type of damage. Say... 5/- vs Slashing or 5/- vs Fire.
Soaking is written 5/+3 or similar and means that it removes 5 physical damage (Slashing, Bludge, Piercing) for weapons that are below +3 (in other words... addy weapons would do 5 less damage while mithril weapons would do damage as usual, if not counting with GMW).