VB.NET book exercise that outputs monthly balances from a fixed amount deposit

I’m going through practice exercises from a book and am stuck on a particular example. Full disclosure, this is not an assignment or test but merely me practicing through chapters of a book called Introduction to Visual Basic so off the bat, I apologize to those who feel it is a stupid question to ask. The question goes like this,

Suppose a fixed amount of
money is deposited at the beginning of each month into an
investment paying 6% interest compounded monthly. After each
deposit is made,

new balance] = 1.005 * [previous balance one month ago] +
[fixed amount].

Write a program that requests the fixed amount of
the deposits as input and displays the balance after each of the
first four deposits.

Here is an attached image of the output

I am not sure how to approach this question really.

  Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
    Dim amount As Decimal
    amount = CDec(txtAmount.Text)
    lstResults.Items.Add(MonthlyInvestment(amount)).ToString("C")
  End Sub

  Function MonthlyInvestment(amount As Decimal) As Decimal
    Dim balance As Decimal = amount
    Dim newBalance As Decimal
    Dim interestRate As Decimal = 0.06
    newBalance = 1.005 * balance + amount
    Return newBalance
  End Function
End Class

You are using a TextBox control which is not the best option for accepting numeric values. My first suggestion would be to swap that out in favor of a NumericUpDown control. You can find the documentation here: https://docs.microsoft.com/en-us/dotnet/api/system.windows.forms.numericupdown

After that what you should do is setup a For/Next loop to loop from month 2 (since month 1 is the initial deposit) to month n. Outside of your loop you should declare a variable to hold the current total and inside of your loop, you should do your calculation based on the current total.

Take a look at this example using a NumericUpDown:

Dim total = NumericUpDown1.Value
lstResults.Items.Add($"Month 1: {total:c}")
For month = 2 To 4
    total = 1.005 * total + NumericUpDown1.Value
    lstResults.Items.Add($"Month {month}: {total:c}")
Next

Fiddle: https://dotnetfiddle.net/2CK432

Also, quick side-note, 1.005 is not 6 percent. Not sure if that’s a typo in your code.

Here’s an example you can refer to.

Private count As Decimal
Private monthCount As Integer = 1
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
    If count = 0 Then
        count = CDec(txtAmount.Text)
        lstResults.Items.Add($"Month {monthCount}:   " + count.ToString("C"))
        monthCount += 1
    Else
        lstResults.Items.Add($"Month {monthCount}:   " + MonthlyInvestment(count).ToString("C"))
        monthCount += 1
    End If
End Sub
Function MonthlyInvestment(balance As Decimal) As Decimal
    count = 1.005 * balance + CDec(txtAmount.Text)
    Return count 
End Function

Result of my test.

Hi all thank you for your help. I went back and looked at my previous work and indeed as one said, breaking down these problems I was able to see a pattern. This is what I did

Public Class Form1
Dim balance As Double = 0 'global variable, We need to access it everywhere in the project
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
Dim originalDeposit As Double = CDbl(txtAmount.Text)
'clears the listbox and asaaign 0 to balance when the new blance is being entered
lstResults.Items.Clear()
balance = 0

'  Option 1 displays results

'lstOutput.Items.Add("Month 1: " & newBalance(originalDeposit).ToString("C"))
'lstOutput.Items.Add("Month 2: " & newBalance(originalDeposit).ToString("C"))
'lstOutput.Items.Add("Month 3: " & newBalance(originalDeposit).ToString("C"))
'lstOutput.Items.Add("Month 4: " & newBalance(originalDeposit).ToString("C"))

' Option 2 - displays results

For i As Integer = 1 To 4
  lstResults.Items.Add("Month " & i & ": " & newBalance(originalDeposit).ToString("C"))
Next

End Sub
‘calculates new balance. This function is called 4 times. It increase the balance
‘and returns its new value
Function newBalance(originalDeposit As Double) As Double
balance = 1.005 * (balance) + originalDeposit
Return balance
End Function

End Class