How are multiple damage reductions on the same creature handled? Do they "stack" or do they overlap? I am assuming that if If I have damage reduction 15/silver and 10/+1 then a silver weapon would still face the 10/+1 reduction. But would a mundane non-silvered weapon have to deal with 15 or 25 damage reduction?
Two or more kinds of damage reduction do not stack. However, a creature with multiple damage reduction qualities gets the benefit of the best one for any given attack against it. Against a silver weapon, the sample creature would still have 10 points of DR. Against a mundane weapon the creature would a DR of 15.