DiscountApplier.cs

public class DiscountApplier
{
    public int GetDiscountPercentageValue(Product[] products) 
    {
        var discountValue = 0;
        if (products.Length == 1) {
            return 0;
        }

        var allProductsQuantity = products.Sum(p => p.Quantity);

        if (allProductsQuantity is > 10 and <= 50) {
            discountValue += 10;
        }
        if (allProductsQuantity > 50) {
            discountValue += 15;
        }

        var allProductsPrice = products.Sum(p => p.Quantity * p.Price);

        if (allProductsPrice is > 100 and <= 500) {
            discountValue += 5;
        }
        if (allProductsPrice is > 500 and <= 2000) {
            discountValue += 10;
        }
        if (allProductsPrice > 2000) {
            discountValue += 25;
        }

        return discountValue;
    }
}