I'm not sure if it's simple, but several games have different ways they implement the interaction between armor and damage.
For example:
damage = 15
hp = 100
armor = 10
#no armor
hp = hp - damage
>hp: 85
#with armor1
damage = damage - armor
damage = clamp(damage, 0, max_damage) #prevents having negative values
hp = hp - damage
>hp: 95
#with armor2
damage = damage - (damage * (armor/max_armor)) #assuming max_armor = 99
hp = hp - damage
>hp: 86.51