Items discount

Hi

I use NAV V3.70B

In Item card, each item is part of Item Disc Group

Want to be able to discount customer or customers as this example: Item A,B,C all part of same group.

Buy 50+ of any or combination of items A,B,C get 5% disc
Buy 250+ of any or combination of items A,B,C get 10% disc

How can I do that ?

Thanks in advance