You need:
-80% overall
-90% average across your A2 modules (say you've got two A2 modules, you could get 80% in one and 100% in the other)
-For maths, you need 90% average across C3 and C4 rather than all 3
So if you get 100% at AS and 89% at A2 you'll get an A, but if you get 70% at AS and 90% at A2 you'll get an A*.