if (combobox1.SelectedValue.ToString() == "Regular")
    {
        x = 1.25;
    }
else if (combobox1.SelectedValue.ToString() == "Double")
    {
        x = 2;
    }
 // Same Codes for ComboBox 2~5

if (txtTime.Text.Lenght > 0)
    {
    // Item Letter "C"
    // value of "x" is equal to the above item
    txtAmount.Text = (double.Parse(txtTime.Text) * x).ToString();
    }