Tasks are at different level of difficulties. For each task you need to write a method with your solution. What's more, you also need to call your method with different input and check if returned result is OK.
For LINQ there is different story. You don't need to write a separate method with solution, query is enough, but you have to test it to check if returns correct result.
You can prepare solutions in your favourite IDE (e.g. Visual Studio) or online compiler (e.g. Ideone).
Enjoy!