For real fun, use Excel:<p><pre><code>    =0.1+0.2-0.3        =&gt; 0

    =(0.1+0.2-0.3)      =&gt; 5.55112E-17
</code></pre>
(inspired by <a href="http:&#x2F;&#x2F;www.cs.berkeley.edu&#x2F;~wkahan&#x2F;Mindless.pdf" rel="nofollow">http:&#x2F;&#x2F;www.cs.berkeley.edu&#x2F;~wkahan&#x2F;Mindless.pdf</a>)