Can we assume that this is part of a userform? It is not clear why you need two list boxes. Your message suggests that you should use a list box and a text box, thus
Code:
Private Sub ListBox1_Click()
Dim sMyStr As String
sMyStr = ListBox1.value
If sMyStr = "P21" Then
TextBox1.Text = "41"
ElseIf sMyStr = "P69" Then
TextBox1.Text = "31"
ElseIf sMyStr = "P62" Then
TextBox1.Text = "11"
ElseIf sMyStr = "P64" Then
TextBox1.Text = "42"
Else
TextBox1.Text = ""
End If
End Sub
Private Sub UserForm_Initialize()
Dim i As Integer
With ListBox1
For i = 21 To 69
.AddItem "P" & i
Next i
End With
End Sub