how initialize a variable with alternative values

Thanks for the response however I think I figured it out.
 
In my CMyDlg.cpp
 
I initialized the variableA = 80
 
I then used ON_EN_UPDATE for the Edit control.
 

void CMyDlg::OnEnUpdateEDITVARIABLEA()
{
UpdateData();

VariableA = 100 - VariableB;


UpdateData(FALSE);
 
}
 
This way the edit control shows 80 for VariableA when I open up the dialog, but when I change VariableB it shows the value of 100 - VariableB.