If you see "Buy 5 get 10% off" then this is the marketing text for Quantity Discounts, not Better Together.
Your other question suggests you may not be clear about what a master category is. Although categories have subcategories, products only have one master category, and only master category numbers may be used with Better Together. If you have category 5 with subcategories 6 and 7, you can only use 6 and 7 in Better Together; category 5 will not be the master category of any product (since it contains subcategories).
This is the documentation I have provided on category handling:
http://www.thatsoftwareguy.com/zenca...ry_issues.html
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