In the rule 'total pages = $32b$', where $b$ is the number of booklets printed, which quantity stays the same no matter how many booklets are made?

Imagine you want to describe 'a number doubled, plus three'. Without algebra you would have to make a list: $2\times 1+3=5$; $2\times 2+3=7$; $2\times 5+3=13$; … forever. With a letter — say $x$ for 'the number' — the whole list becomes one short expression: **$2x+3$**. Pick any value of $x$ and you get the right answer. The letter is shorthand for 'whatever number we're talking about'.