I have $11$ choices for the first book and $10$ choices for the second book, making $11\cdot 10$ pairs. But each pair has been counted twice (once for each ordering of the two books). Since order doesn't matter, the actual number of pairs I can choose is $(11\cdot 10)/2$, which is $\boxed{55}$.
I have eleven books, of which I want to bring two to read on vacation. How many different pairs can I choose?
{'answer': '55', 'subject': 'Prealgebra', 'level': 3, 'unique_id': 'train/prealgebra/986.json'}