Just a bit of textbook blithering tabloid innumeracy for you - today's Daily Mail contains this headline:
That image is from the front page of the Mail's website; the actual article has had its headline amended to something that at least makes sense.
The interesting thing with these is always not so much to spot the error - that's easy - but more to try to determine what the person who wrote it was actually thinking, given that they'd presumably done a calculation of some sort to get hold of the numbers. The sort of thing you want to be doing as a sanity check here is to say to yourself: if you had a thing at a certain price, and you reduced that price by 100%, it would end up being free. So if my calculation gives any number greater than 100, I've clearly cocked it up somewhere.
In this particular case, a £30 bottle of wine being offered for £7 represents a discount of (23 / 30) * 100 = 77%. I can't actually imagine a way of carving the numbers up to get a figure of 200, but on the face of it a £30 bottle of wine with a 200% discount means Asda paying customers £30 to take each bottle away. If I really thought that was true I'd be down there RIGHT NOW.