Nowhere do you ever check whether both boxes are checked and add 10. You display either (dblMon) or (dblMon + 5.0) but nowhere do you display (dblMon + 10.0). Start with dblMon, test the first box and add 5.0 if it's checked, test the second box and add 5.0 if it's checked and then display the result. Put that in a method and call it from both event handlers.