PickNthElement.cs

// Pick every 10th element:
int n = 10;

// Range of numbers:
List<int> numbers = Enumerable.Range(1,100).ToList();

// List of nth elements:
List<int> nthElements = new List<int>();

// Pick every 10th element:
Enumerable.Range(0, numbers.Count()/n)
    .ToList()
    .ForEach(k => nthElements.Add(numbers.Skip(k*n).First()));

// Outputs the results:
nthElements.Dump("10th elements");