private void displayNextProblem()
{
//get the next problem
_CurrentProblem = ProblemFactory.GetArithmeticProblem <int>(getNextOperation(), 2, 20, false);
string[] lines = _CurrentProblem.Display().ToArray();
if (lines.Length > 0)
{
lines[0] = lines.First().Remove(0, 1); //remove the first symbol
}
ProblemBox.Lines = lines;
AnswerBox.Text = string.Empty;
AnswerBox.Focus();
_Timer.Restart();
}