This must be easy for you old pros ;-)
I have a product which I want to offer at 10% discount for every 2 bought, so:
1 in cart = full price2 in cart = (2 x price) - 10%3 in cart = ((2 x price) - 10%) + price4 in cart = 2 x ((2 x price) - 10%)
5 in cart = 2 x ((2 x price) - 10%) + price etc.
For every pair, price is 10% off the pair and if the no is odd price is that plus the full one for this single remainer.