Configure so different attributes/child products share stock virtuemart 2.0

[NJ_]

Hello
I just started out with a new website based on joomla 2.5/virtuemart 2.0.6 and i sell different packs of same product 12,50 and 100. So i need to set up so three different stockable variants share the same stock but with three different quantities of it. So if i have 100 in stock pepole could buy 8x12,2x50 or 1x100.

How do i set this up? I appreciate all the help I can get.

//NJ_