Sets

\[\xi=\{1,2,\ldots,24\}.\]\(A\) is the set of multiples of \(3\) in \(\xi\).
\(B\) is the set of factors of \(24\).

1. List \(A\) and \(B\).

2. Find \(A\cap B\).

3. Find \(n(A\cap B)\).

4. A student includes \(9\) in \(A\cap B\). Explain the error.