Combination/Permutation Problem I'm having trouble understanding this combo/perm problem. The answer is supposed to be 12. Can someone please explain how they got it? You need to put your reindeer, Prancer, Quentin, Rudy, and Jebediah, in a single-file line to pull your sleigh. However, Rudy and Prancer are best friends, so you have to put them next to each other, or they won't fly.

Will just refer to them by their first letter.

But we basically just consider "RP" as a reindeer. So how many ways are there of ordering RP, Q, J? 3 choices for RP, 2 for Q and 1 for J giving us a total of 6. But the ordering "RP-- "is equivalent to ordering "PR--" so you'd just double your first quantity giving you $$6\cdot 2=12.$$