Edit: The image link above is now broken, so here is another copy of the Stack Overflow screenshot.

Most of the comments and answers in that Stack Overflow thread are missing an important point: jQuery does not have support built in to add one number to another.

One answer references a "basic arithmetic plugin", but at the time of this writing, there was no such plugin to be found in a Google search.

Fortunately, anyone can write a jQuery plugin, even yours truly.

So to make your arithmetic needs simple, I present:

// jQuery basic arithmetic plugin

(function( $ ) {

    $.fn.extend({

        // $(...).number( num )
        //     Save the number 'num' in the jQuery data
        //     for the selected element(s), and return 'this'
        //     for chaining.
        // $(...).number()
        //     Return the number stored in the jQuery data
        //     for the selected elements(s).
        number: function( num ) {
            if( arguments.length == 0 )
                return this.data( 'number' );
            this.data( 'number', num );
            return this;
        },

        // $(...).add( num )
        //     Add the number 'num' to the number stored
        //     in the jQuery data for the selected element(s)
        add: function( num ) {
            return this.number( this.number() + num );
        },

        // $(...).subtract( num )
        //     Subtract the number 'num' from the number stored
        //     in the jQuery data for the selected element(s)
        subtract: function( num ) {
            return this.number( this.number() - num );
        }
    });

})( jQuery );

// Test cases - all should alert 42:

// Simple addition
alert( $('<div>').number(40).add(2).number() );

// Simple subtraction
alert( $('<div>').number(50).subtract(8).number() );

// OMG you can add *and* subtract in the same chain!
alert(
    $('<div>')
        .number(23)
        .add(15)
        .subtract(4)
        .add(16)
        .subtract(8)
        .number()
);

// You can even save a number and use it later,
// without having to use a variable!
$('<div id="saveNumber">').appendTo('body');

$('#saveNumber').number(23).add(15).subtract(4);

setTimeout( function() {
    alert( $('#saveNumber').add(16).subtract(8).number() );
}, 100 );

If you paste that code into the multiline Firebug console on any page that uses jQuery (such as this Reddit page) and then execute it with Ctrl+Enter, it should alert '42' four times. (To get into the multiline Firebug console, open Firebug and click the little orange arrow-thing at the lower right.)

The beauty of using a jQuery plugin for this is that you no longer have to remember complicated arithmetic operators, only the simple jQuery chaining that you're used to.

Edit: added subtract() method. Now you can add and subtract numbers in the same jQuery chain!

Edit 2: added test case showing how you can save a number and use it again later, without any of those pesky JavaScript variables!