That boy must have received a lot of PMs...
I can confirm DC's answer: 15/- cold resist and 5/- cold resist do not stack. In general, any two effects that have an identical description do not stack, instead having the highest take effect and the lowest being redundant.
You have 5 types of "defensive effects" against physical damage, all of which use the best effect within their category which is applicable for the attacking effect:
1) physical damage resistance, f.i. Brawler's Belt (5/- damage resistance to bludgeoning)
2) physical damage reduction, f.i. Stoneskin spell (10/+5 vs. all physical) or monk's resistance (20/+1 vs. all physical)
3) physical damage soak, f.i. Adventurer Robe (+1 soak 5 damage, effectively 5/+1 damage soak)
4) elemental damage resistance, f.i. Cloak of Fire Resistance (15/- resistance to fire)
5) elemental damage reduction, f.i. Protection from Elements spell (30/- resistance to all elements)
Of course within the elemental resistance group there are five subgroups, one for each element, so 20/- fire resist and 20/- cold resist "stack" and both work against the same hellball coming your way.
Damage Resistance and Soak come before Damage Reduction, which means that damage resistance will make your damage reduction effects (that are often temporary spelleffects that wear down after a set amount of damage has been reduced, only exception being the inate reduction of monks and other special creatures) last longer.
In your example, 20/+1 is group 2, 15/- cold resist is group 4 (cold) and 5/- cold resist is group 4 (cold), so 5/- cold is superseded by the 15/- cold resist.
As a final note: although the different effects of damage reduction do not stack, they DO work in sequence because they have durations: if you have both a Stoneskin (10/+5 reduction) and a Premonition (30/+5 reduction) active, initially only the premonition works. However, unless the duration of the Stoneskin has lapsed, it will still be active when the Premonition has absorbed it's maximum amount of damage and it will kick into action the first attack
after the Premonition has stopped working. The same goes for spells like Protection from Elements and Resist Elements.
Hope this helps!

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