To get that URL you will need Joomla!'s SEF/SEO settings turned on (I'm assuming you've already got that setup).
In Joomla you have to create a new menu, call it something like "Utility Menu" but don't give it a "Module Title" - you don't want to display this menu so you don't need the module.
Then create a "Single Article" menu item with the alias 15k
as a menu item within the "Utility Menu". You should now be able to access the article at the http://www.yoursite.com/15k
If you want to link to this from another menu create a menu item alias and point it at the 15k menu item. If you want to link from within an article simply point the link to the 15k menu item.
[Edit]
Routing in Joomla! first looks through the user defined paths (i.e. menu's and their menu items) then if it can't find a match for the requested resource that way it asks the particular component to build the route and if the components router is basic (i.e. nearly every single one in Joomla! 1.5) then you get the ?option=com_somecomponent&id=X
style result. In 2.5 this is much improved but still uses the same routing mechanism.