function calculate_average() {

                var english_marks = parseInt(document.getElementById("txt1").value);
                var prog_marks = parseInt(document.getElementById("txt2").value);
                var maths_marks = parseInt(document.getElementById("txt3").value);
                var average = (english_marks + prog_marks + maths_marks)/3; ;

                document.getElementById("txt4").value = average;
        }