Put this code In a module
Function PAYBACK(invest, finflow)
Dim x As Double, v As Double
Dim c As Integer, i As Integer
x = Abs(invest)
i = 1
c = finflow.Count
Do
x = x  v
v = finflow.Cells(i).Value
If x = v Then
PAYBACK = i
Exit Function
ElseIf x < v Then
P = i  1
Z = x / v
PAYBACK = P + Z
Exit Function
End If
i = i + 1
Loop Until i > c
PAYBACK = "no payback"
End Function
