[jQuery] Autoselect a checkbox on user's input value

[ciupaz]

Hi all,
I have two checkboxes like these ones:

<span>
    <input name="place" id="internal" type="checkbox"
value="internal" />Internal (up to DN 40)<br />
    <input name="place" id="external" type="checkbox"
value="external" />External (over DN 40)
</span>

and a textbox like this:

Insert the DN number:
<input name="DNnumber" id="DNnumber" type="text" />

I'd like a sort of validation that when the user insert a DN number
greater than 40, the checkbox "external" will be automatically
selected (without change it).

How can I accomplish this?

Thanks a lot.

Luigi