The latter, armor absorbs damage first, then you hit the creature, where the resistance reduces the remaining damage. So 20 damage -3 and then -50% to 9 damage, as it is rounded up, not down, as says on page 84 (As a rule of thumb, it is always rounded up)Yes, but if a monster that has resistance is also wearing armour do you apply it’s resistance damage reduction first (-50%) then deduct the armour rating, or the other way around as it will give different results in how much damage is applied to its HP.
For example 20 damage…
-50% resistance = 10, then
-3 armour rating = 7 damage
Or,
-3 armour rating = 17, then
-50% resistance = 8.5 damage (rounded down to 8)