A taki np. zapis
[ on-line ]
(double.Parse(value) / 10).ToString("0.0")
string value = "5002"; // wartość 5002 to 500.2
Console.WriteLine((double.Parse(value) / 10).ToString("0.0"));
lub
[ on-line ]
string value = "5002";
Console.WriteLine(value.Insert(value.Length - 1, "."));
BTW,
string.Format("{0:0.0}", double.Parse(value) / 10)