Cant figure out discounts

Trying to set up a store for my business. I'm new to Zen Cart.

I sell trophies and plaques. We offer qty. discounts on purchases of 20, 50 and 75.

What i'm trying to see if it's possible, say someone buys 20 trophies, but combines different ones, how or can I set up qty. discounts for them combined? Same thing with plaques etc etc. Like categories can combine for qty. discounts.

Say 15 of one and 10 of another would get a discount. I started settting it up and swear I had it working once.

Thanks in advance, hope I explained this correctly.
Tony

If the trophies are different product id's, you must use the Quantity Discounts contribution; you can only use the native quantity discounter to discount quantities of a single product.