Private Sub TextBoxA_KeyPress(KeyAscii As Integer)
  ClearTextBox
End Sub
 
Private Sub TextBoxA_Validate(Cancel As Boolean)
  DoCalculation
End Sub
 
Private Sub DoCalculation()
  If TextBoxA.Text <> "" And TextBoxB.Text <> "" Then
    TextBoxC.Text = Val(TextBoxA.Text) + Val(TextBoxB.Text)
  ElseIf TextBoxA.Text <> "" And TextBoxC.Text <> "" Then
    TextBoxB.Text = Val(TextBoxC.Text) - Val(TextBoxA.Text)
  ElseIf TextBoxB.Text <> "" And TextBoxC.Text <> "" Then
    TextBoxA.Text = Val(TextBoxC.Text) - Val(TextBoxB.Text)
  End If
End Sub
 
Private Function ClearTextBox()
  If TextBoxA.Text <> "" And TextBoxB.Text <> "" And TextBoxC.Text <> "" Then
    TextBoxA.Text = ""
    TextBoxB.Text = ""
    TextBoxC.Text = ""
  End If
End Function
 
Private Sub TextBoxB_KeyPress(KeyAscii As Integer)
  ClearTextBox
End Sub
 
Private Sub TextBoxB_Validate(Cancel As Boolean)
  DoCalculation
End Sub
 
Private Sub TextBoxC_KeyPress(KeyAscii As Integer)
  ClearTextBox
End Sub
 
Private Sub TextBoxC_Validate(Cancel As Boolean)
  DoCalculation
End Sub

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