hitung rpn

// for hitung rpn --------------------------------------
        var a = opener.document.getElementById('grade').value;
        var b = opener.document.getElementById('rank1').value;
        var c = opener.document.getElementById('rankdet1').value;
        var hitung = a * b * c;
        opener.document.getElementById('rpn1').value = hitung;

// end hitung rpn1---------------------------------------