gistfile1.cs

/// <summary>
/// Solution for calculate xy=total, x-y -> 0.
/// Reference: http://www.wolframalpha.com/input/?i=xy%3D200,+x%3E0
/// </summary>
/// <param name="total">The integer to calculate.</param>
/// <returns>x,y pair, x &lt;= y.</returns>
public static KeyValuePair<int, int> MinXPlusY(int total)
{
    var allXplusY = Enumerable.Range(1, total)
        .Where(i => total % i == 0 && i <= total / i)
        .ToDictionary(i => i, i => total / i);

    var min = allXplusY
        .Select(d => d.Value - d.Key)
        .Min();

    //xy is what you finding.
    return allXplusY.FirstOrDefault(d => d.Value - d.Key == min);
}