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Perform maths function on number input

Hi im completely new to coding so forgive me for being a complete noob but Im looking for a script that will allow me to have 2 input boxes (one for grams of fat the other for grams of sugar) and on entry I need it to display the first amount divided by 300 and the second divided by 4 in two text boxes?

Not pretty, but shows you what to do:

Code:

<!DOCTYPE html>

<html>

<head>

<title>Title of the document</title>

</head>

<body>

Enter grams of fat:

<input type="text" id="fatin" onkeyup="document.getElementById('fat').value = document.getElementById('fatin').value/300;">

This is a changed fat thing: <input id="fat"><br />



Enter grams of sugar:

<input type="text" id="sugin" onkeyup="document.getElementById('sugar').value = document.getElementById('sugin').value/4;">

This is a changed sugar thing: <input id="sugar">

</body>

</html>

Did not check for numbers only.