Visual Basic Programming Help?

I have to make an auto loan calculator, I created the basic code, but my professor want us to change the interest rate depending on the credit score of the buyer. I am not sure how to do it.

Here's what our professor asked us to do:

1. Amount of the loan (vehicle coast)

2. Down Payment

3. Number of months

4. New or Used Car

5. Annual interest rate, the interest rate is a function of the credit score of the buyer

a. credit score of 700-699 interest rate is 2%

b. credit score of 600-699 interest rate is 3%

c. credit score of 500-599 interest rate is 4%

d. credit score of 400-499 interest rate is 5%

e. credit score less than 400 interest rate is 10%

6. If the car is a new car than 0.5% will be added to the interest rate

And here is my code.

Thanks for the help.

Public Class Form1

'Class Level Constants

Const dblMONTHS_YEAR As Double = 12 'Months Per Year

Const dblNEW_RATE As Double = 0.05 'IR, New Car

Const dblUSED_RATE As Double = 0.08 'IR, Used Car

'Class Varible to hol the annual interest rate

Dim dblAnnualRate As Double = dblNEW_RATE

Private Sub btnCalculate_Click(sender As Object, e As EventArgs) Handles btnCalculate.Click

Dim dblVehicleCost As Double 'Cost

Dim dblDownPayment As Double 'Down Payment

Dim intMonths As Integer '# of months for the loan

Dim dblLoan As Double 'Amount of loan

Dim dblMonthlyPayment As Double 'Monthly Payment

Dim dblInterest As Double 'Interest Paid for the period

Dim dblPrincipal As Double 'Principle paid for the period

Dim intCount As Integer 'Counter for the loop

Dim strOut As String ' Used to hold a line of output

Dim blnInputOk As Boolean = True

'Get the vehicle cost, validating at the same time.-

If Not Double.TryParse(txtCost.Text, dblDownPayment) Then

lblMessage.Text = "Vehicle cost must be a number."

blnInputOk = False

End If

'Get the down payment, validating the same time.

If Not Integer.TryParse(txtMonths.Text, intMonths) Then

lblMessage.Text = "Months must be an integer."

blnInputOk = False

End If

If blnInputOk = True Then

'Calculate the loan amount and monthly payment

dblLoan = dblVehicleCost - dblDownPayment

dblMonthlyPayment = Pmt(dblAnnualRate / dblMONTHS_YEAR, intMonths, -dblLoan)

End If

'Clear the list box and message label.

ListBox1.Items.Clear()

lblMessage.Text = String.Empty

For intCount = 1 To intMonths

'Calculate the interes rate for this period.

dblInterest = IPmt(dblAnnualRate / dblMONTHS_YEAR, intCount, intMonths, -dblLoan)

'Calculate the principal for this Period.

dblPrincipal = PPmt(dblAnnualRate / dblMONTHS_YEAR, intCount, intMonths, -dblLoan)

'Start Building the output string with the month.

strOut = "Month" & intCount.ToString("d2")

'Add the payment amount to the output string.

strOut &= ":payment =" & dblMonthlyPayment.ToString("n2")

'Add the interest amount to the output string.

strOut &= ", interest =" & dblInterest.ToString("n2")

'Add the principal for the period.

strOut &= ", principal =" & dblPrincipal.ToString("n2")

'Add the output string to the list box

ListBox1.Items.Add(strOut)

Next

End Sub

Private Sub btnClear_Click(sender As Object, e As EventArgs) Handles btnClear.Click

radNew.Checked = True

dblAnnualRate = dblNEW_RATE

lblAnnualRate.Text = dblNEW_RATE.ToString("p")

txtCost.Clear()

txtDownPayment.Clear()

txtMonths.Clear()

ListBox1.Items.Clear()

lblMessage.Text = String.Empty

txtCost.Focus()

End Sub

Private Sub btnExit_Click(sender As Object, e As EventArgs) Handles btnExit.Click

Me.Close()

End Sub

Private Sub radNew_CheckedChanged(sender As Object, e As EventArgs) Handles radNew.CheckedChanged

If radNew.Checked = True Then

dblAnnualRate = dblNEW_RATE

lblAnnualRate.Text = dblNEW_RATE.ToString("p")

ListBox1.Items.Clear()

End If

End Sub

Private Sub radUsed_CheckedChanged(sender As Object, e As EventArgs) Handles radUsed.CheckedChanged

If radUsed.Checked = True Then

dblAnnualRate = dblNEW_RATE

lblAnnualRate.Text = dblUSED_RATE.ToString("p")

ListBox1.Items.Clear()

End If

End Sub

End Class