To better understand LINQ you will need to have beginner level familiarity with these terms:

1. Generic Collections (System.Collections.Generic): https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/concepts/collections#BKMK_SimpleCollection
2. Lambdas https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/operators/lambda-expressions
3. Delegate https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/delegates/
4. Extension Methods https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/classes-and-structs/extension-methods

https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/concepts/linq/
https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/concepts/linq/introduction-to-linq-queries

In this repository you will find some basic exercises to try and solve on your own. For a reference on operators you can use - please look here: https://docs.microsoft.com/en-us/dotnet/api/system.linq.enumerable?view=net-5.0

You can only modify the "Act" part of a unit test. The unit test must pass.

https://app.pluralsight.com/library/courses/linq-fundamentals-csharp-6
https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/concepts/linq/standard-query-operators-overview
https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/concepts/linq/classification-of-standard-query-operators-by-manner-of-execution
https://github.com/dotnet/try-samples/blob/main/101-linq-samples/readme.md