Amy is organizing her bookshelves and finds that she has 10 different…

Amy is organizing her bookshelves and finds that she has 10 different types of books. She then codes each book with either a single letter or a pair of two different letters. If each type of book is uniquely represented by either a single letter or pair of letters, what is the smallest number of letters Amy will need to create the codes for all 10 types of books? (Assume the order of letters in a pair does not matter.)

AQuA-RAT — Algebra Question Answering with Rationales (Ling et al., DeepMind, 2017) · test #65