The 3 litres of milk are much less expensive than the 6 pints.  Here is how you figure this out.

First, you must know how many pints are in a litre.  The answer is that there are 2.11 pints in a litre.  That means that 3 litres is equal to 6.33 pints.

Next, let us find the price per pint in the example you give.  To do that, we divided 2.10/6.  That tells us that milk is .35 per pint.

So now we multiply that by 6.33 pints to find out how much the 3 litres would cost at that rate.

.35 times 6.33 is 2.21.

That means that if we paid this price for 6 litres of milk, we would pay 2.21 instead of 1.80.

And I'm sorry, but as an American I don't know how to type the pound sign...