Managing prepacked products with price per unit of measure

We have a project where product price is expressed per kg of product.
However, product is prepacked and it can be sold only as a whole.

For example:
A pack of 7 bananas weights about 1.5 kg, price is expressed as EUR 2 per 1kg.
So, in the shopping cart quantity should be 1 piece and a price 3 EUR.

What would be suggested approach here?