Let's call the stamps A, B, and C. They can each be used only once. I assume all 3 must be used in each possible arrangement. There are two ways to solve this. We can list each possible arrangement of stamps, or we can plug in the numbers to a formula. Let's find all possible arrangements first. We can easily start spouting out possible arrangements of the 3 stamps, but to make sure we find them all, let's go in alphabetical order. First, let's look at the arrangements that start with A: ABC ACB There are no other ways to arrange 3 stamps with the first stamp being A. Let's look at the ways to arrange them starting with B: BAC BCA Try finding the arrangements that start with C: C_ _ C_ _ Or we can try a little formula; y×(y-1)×(y-2)×(y-3)...until the (y-x) = 1 where y=the number of items. In this case there are 3 stamps, so y=3, and the formula looks like this: 3×(3-1)×(3-2). Confused? Let me explain why it works. There are 3 possibilities for the first stamp: A, B, or C. There are 2 possibilities for the second space: The two stamps that are not in the first space. There is 1 possibility for the third space: the stamp not used in the first or second space. So the number of possibilities, in this case, is 3×2×1. We can see that the number of ways that 3 stamps can be attached is the same regardless of method used.
