Do Rings of Elemental Resistance stack?

If you wear two rings of Elemental Resistance, do they stack? Because it appears that my DR is only absorbing 15 points of acid, cold, electrical, and fire damage.
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I know, but Elemental Immunity tends to be a fairly rare event for players, usually being given by custom magic items. Likewise, some monsters have elemental immunity, but only to a certain percentage, so my earlier post was musing on what would happen in circumstances where, say, you might have a creature that was 50% immune to fire, and then wears an Amulet of Fire Resistance (20/fire). If they were then hit by a Maximised Fireball (60 fire damage), what would be the final damage total it would take? Would it be 10 (immunity goes first, reduces damage to 30, -20 from the amulet = 10), or 20 (resistance goes first, reduces damage to 40, halved by immunity = 20)?