Is there any damage type that cannot be resisted?
In other words, if each of your resistances provides 20% damage reduction from it's respective damage type, is it the same as having 20% damage reduction from armor?
Solution 1:
There is no damage type that cannot be resisted, so yes, 20% reduction from resistances is equivalent to 20% reduction from armor.
And as fbueckert pointed out, damage reduction is taken sequentially, not additively.