try-and-catch

It's not impossible.  If you are using a Console application with C# 2005, you can simply clear the screen with:

Console.Clear();

You can also use the TryParse method instead of a try-catch block:

string sa; double da;
Console.Write("Enter a number:  ");
sa = Console.ReadLine();
Double.TryParse(sa, out da);
Console.WriteLine("{0} parsed to a Double is:  {1}", sa, da);

If the input is non-numeric, it will simply return 0, without an error.  If the user intended it to be a zero, you can test if his string input was "0" otherwise inform him to reenter.

It's also possible to control the cursor location in a Console application (starting with NET framework 2.0) with Console.CursorTop and Console.CursorLeft, then erase the text following that location by overwriting it with spaces, and then returning to the same location at the beginning of the input.

it works but what i want to happen is whenever the user inputs an invalid answer, it will just erase what he has input, meaning the cursor will go back to its previous location.

Your catch block really does not return value. The function must return a value even if an error occures. In your case it will be solution to return double.NaN value (see docs for more info) after informing user about "invalid input" :)

Example:

static double getQA()
{
   try {
      double qA = Convert.ToDouble(Console.ReadLine());
      return qA * 0.10;
   }
   catch {
      Console.WriteLine("Invalid Input!");
      return double.NaN;
   }
}