Group and Total with LINQ

var products = new[]
{
    new Product("Keyboard", "Accessories", 25m, 10),
    new Product("Mouse", "Accessories", 10m, 20),
    new Product("Monitor", "Displays", 150m, 4),
};

foreach (var group in products.GroupBy(p => p.Category).OrderBy(g => g.Key))
    Console.WriteLine($"{group.Key}: {group.Sum(p => p.Price * p.Quantity)}");

public record Product(string Name, string Category, decimal Price, int Quantity);