How are attack modifiers determined for multiclass?

I'm assuming it's not addition (because that would get broken fast)

Level 5 character with Fighter 1 Barbarian 1 Cleric 1 Rogue 1 Paladin 1 obviously shouldn't get +5

I'm guessing it'll be some kind of weighted average? 

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Use the one you have most levels in, or Use whichever is lowest of your classes if tied
We're not sure. There's talk of a multiclassing system with the "first level" class features spread over three levels, and most groups starting at Level 3 (based on the latest Legends and Lore), but we don't know yet.