Obviously we need to turn over the D to check that there is a 3 on the back (everybody gets this one right). And equally obviously, there's no need to turn over the K (and again, everybody realises this). The 3 card is a tricky one. Most people think that you need to turn this card over to see whether there is a D on the other side. This would be necessary had the claim been that "Every card that has a D on one side has a 3 on the other, and vice versa". But it wasn't. The 7 is the other tricky one. It doesn't occur to most people that we need to turn this card over to check that the letter on the back is not D. If it is D, then the claim is false.