Solution
= Solution
The <rule of product> says that a sequence of $k$ choices, with $n_i$ possibilities at stage $i$ regardless of earlier choices, has \b[$\prod_{i=1}^k n_i$] possible outcomes. Applied to the <Cartesian product> of <finite sets>, it gives $|A\times B|=|A||B|$. Here independence means that the available choices do not restrict one another; no <probability> assumption is needed.