Quantity for variable product

Good afternoon,

I’m trying to use Woocommerce for the first time (on a local host).

I set a variable product, with price changing depending on weight (there are packs containing 30g, 50g or 100g of product). I would like also to give the user the opportunity to buy more packs of the same quantity (e.g. 2 boxes of 100g), but I cannot find the option to do it.

Can anyone help me?

Thanks in advance

(Hope my English was good enough to explain my problem clearly)