From page 164 of the PHB (here is the equivalent section of the Basic Rules):
Spell Slots. You determine your available spell slots by adding together all your levels in the bard, cleric, druid, sorcerer, and wizard classes, half your levels (rounded down) in the paladin and ranger classes, and a third of your fighter or rogue levels (rounded down) if you have the Eldritch Knight or the Arcane Trickster feature. Use this total to determine your spell slots by consulting the Multiclass Spellcaster table.
If I have a level 3 ranger and multiclass into paladin, how are my available spell slots calculated?
- (ranger/2, rounded down) plus (paladin/2, rounded down)
- (ranger + paladin)/2, rounded down
Which one is it?