Start without thinking of the options. You could do process of elimination, and that's fine, but to learn this kind of problem it's better to think it all the way through. The problem says you have 112 books, and an indefinite number shelves, s, that each hold sixteen books. You need to fit 112 books into shelves, each with sixteen books. That comes down to 112÷s=16, which is the same as 16s=112.