These lines of code need to be changed:
var nu = Math.round(document.getElementById("nu"));
var de = Math.round(document.getElementById("de"));
var wh = Math.round(document.getElementById("wh"));
var wdn = (wh.value*de.value)+nu.value;
var display = document.getElementById("display");
display.value = wdn+"/"+de.value;
Correct use:
var nu = Math.round(document.getElementById("nu").value);
var de = Math.round(document.getElementById("de").value);
var wh = Math.round(document.getElementById("wh").value);
var wdn = (wh*de)+nu;
var display = document.getElementById("display");
display.value = wdn+"/"+de;
Explanation:
Math.round()
cannot work on the HTML element returned by document.getElementById().
However, it can work on the value contained in that element, which will convert the string to a number type (and also round it).