Option Explicit
Function prob(p) As Boolean
If Rnd() < p Then
        prob = 1
    Else
        prob = 0
End If
End Function

Sub coisa_gira()

Dim it As Integer
Dim n As Double
Dim i As Double
Dim p As Double
Dim x() As Double
Dim estado() As Boolean
Dim fim As Boolean
Dim tmedio As Double
'n = InputBox("numero de elementos")
n = 100000                         'modo manual
ReDim x(1 To n) As Double
ReDim estado(1 To n) As Boolean
p = 0.8     ' probabilidade de sair

For i = 1 To n
x(i) = 1
estado(i) = 0
Next

Do
it = it + 1
fim = 1

For i = 1 To n
    If estado(i) = 0 Then
        If prob(p) = 0 Then
                x(i) = x(i) + 1
                fim = 0
            Else
                estado(i) = 1
        End If
    End If
Next

If fim = True Then Exit Do        'usar fim = 1 dá erro de overflow.. Porquê?? Não sei.
'If it = 10 Then Exit Do
Loop

For i = 1 To n
tmedio = tmedio + x(i)
Next
tmedio = tmedio / n
MsgBox (tmedio)
MsgBox (it)

End Sub