JSFiddle - React, Tailwind, and code Playground

HTML

<p>F: <input type="text" name="f" value="50.0" /></p>
<p>=</p>
<p>m: <input type="text" name="m" /></p>
<p>&times;</p>
<p>a: <input type="text" name="a"  /></p>

JavaScript

var $f = $('input[name=f]');
var $m = $('input[name=m]');
var $a = $('input[name=a]');
$m.on('keyup',function() {
    var f = +$f.val();
    var m = +$m.val();
    $a.val(f/m).toFixed(1);
});
$a.on('keyup',function() {
    var f = +$f.val();
    var a = +$a.val();
    $m.val(f/a).toFixed(1);
});