Quantity based discount challenge

Dear all,

One of the products I sell is a € 3,- product. Because I want my customers to buy at least 2 pieces, I want my first product to be priced at € 4,80.

Every next added product is + € 3,-
(so 2 pieces is € 7,80, 3 is € 10,80 etc)

To make it more complicated I want a percentage discount from the 2nd piece, and then every 5th piece.
so 2-4 pieces: 17,5% discount
so 5-9 pieces: 20% discount
so 10-14 pieces: 22,5% discount

That makes this sum. Examples:
3 pieces: (3*3+1,8) – 17,5% = € 8,91
8 pieces: (8*3+1,8) – 20% = € 20,64

Well thát’s a challenge 😀

I’ve tried different things. The extra € 1,80 is messing with all my tried rules. I really hope someone can help me out 🙂

Thanks in advance,
Syd