Explanation:
I think this might be correct you can see for yourself if you think this is good
solution: 5 packs
let a be plates and b be packs
100a = 5b
= (20*5)a
sorry if confusing I'll say it here
packs is 20 plates and you need 100 plates
just divide the plates you need (100) and the plates per pack (20) and that's how many packs you need to get(5)