Hi,
You have two ways to go about it:
1. You can add several prices to your products. A first price with a minimum quantity of 1 and a unit price of 10 ( or 5 ), and a second price with a minimum quantity of 10 and a unit price of 8.5 ( or 4 ).
Then the unit price of the product in the cart will automatically change based on the quantity of the product in the cart.
2. You can have only one price of 10 ( or 5 ) in your product, and then create a coupon via the menu Orders>Coupons. In it, you can activate the "auto load in cart if possible" setting, enter a percentage of 15 ( or a value of 10 ), a minimum order quantity of 10 and select the product in the "products" setting and activate the "coupon percentage applies to product only" so that the 15% discount is only on that product total amount and not the other products of the cart.
Note that with method 2, you can only have one coupon loaded in the cart at the same time.