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...