So say there is a set of 10 baseball cards and I have 1 of them.
If I were to buy a pack that contained 2 baseball cards, what is the probability of getting at least 1 new card?
The easiest way to solve problems like this is to work out the probability that what you're looking for doesn't happen, and then take the answer away from 1.
So, the probability that the first card is the same as the one you already have is ; and the probability that the second one is also the same is also . Therefore the probability that they are both the same is
And so the probability that at least one is different is .