/// <summary>
/// If we list all the natural numbers below 10 that are multiples of 3 or 5, we get 3, 5, 6 and 9. The sum of these multiples is 23.
/// Find the sum of all the multiples of 3 or 5 below 1000.
/// </summary>
/// <returns>return the answer to the problem</returns>
public string Compute()
{
long numberOfTerms = NumberOfTerms(1, 1000, 3);
long lastTerm = LastTerm(numberOfTerms, 3, 3);
long sum3 = MathLibrary.ArithmeticSeries(numberOfTerms, 3, lastTerm);
numberOfTerms = NumberOfTerms(1, 1000, 5);
lastTerm = LastTerm(numberOfTerms, 5, 5);
long sum5 = MathLibrary.ArithmeticSeries(numberOfTerms, 5, lastTerm);
numberOfTerms = NumberOfTerms(1, 1000, 15);
lastTerm = LastTerm(numberOfTerms, 15, 15);
long sum15 = MathLibrary.ArithmeticSeries(numberOfTerms, 15, lastTerm);
return((sum3 + sum5 - sum15).ToString());
}