Having some error in simple interest calculation

Question

I am doing some error in simple interest calculation logic. I am able to calculate for the first row but not able to make principal = the new amount

Private Sub Calculate_Load(sender As Object, e As EventArgs) Handles Me.Load
    Dim p As Double = 50000
    Dim r As Double = 12.5
    Dim cf As Integer = 1
    Dim t As Integer = 12
    Dim a As Double = calcAmount(p, r)

    For index As Integer = 1 To t
        DataGridView1.Rows.Add(index, p, r, a)
        'changing p to new p
        p = a
    Next
End Sub
Function calcAmount(ByVal p As Double, ByVal r As Double) As Double
    Return p + p * r / 100
End Function

Answer 1

As suggested by LarsTech in the comments, you probably wanted INSIDE your loop:

For index As Integer = 1 To t
    DataGridView1.Rows.Add(index, p, r, a)
    'changing p to new p
    p = calcAmount(p, r)
Next