How it works
The first order is paid and its amount is 10.50. The second is paid and its amount is 2.25. The third has an amount of 100 and IsPaid is false, so it stays out. The paid total is 12.75.
A working example
var orders = new[] { new Order("a", 10.50m, true), new Order("b", 2.25m, true), new Order("c", 100m, false) };
var paid = orders.Where(o => o.IsPaid).Sum(o => o.Amount);
Console.WriteLine(paid);Expected output
12.75
Try this
Try the filter on a smaller list.
Try the filter on a smaller list.