public string SolveEfficient()
{
// efficient O(1) technique
const int max = 999;
// max variable is captured by the sumDivisibleBy closure
Func <int, int> sumDivisibleBy = divisor => Maths.ArithmeticSeries(divisor, divisor, (max / divisor));
var sum = sumDivisibleBy(3) + sumDivisibleBy(5) - sumDivisibleBy(15);
return(sum.ToString());
}