21. Write 33 as the sum of $n$ consecutive natural numbers, when $n$ takes the maximum value, change all “ + ” signs in the sum to “ $\times$ ” signs, what is the product then?
【Analysis】According to the problem, $3+4+5+6+7+8=33$, after changing all “ + ” signs in the sum to “ $\times$ ” signs, the product can be found.
【Solution】Solution: According to the problem, $3+4+5+6+7+8=33$,
$$
\therefore 3 \times 4 \times 5 \times 6 \times 7 \times 8=3960 \text {. }
$$
21. Express 33 as the sum of $n$ consecutive natural numbers, when $n$ takes the maximum value, change all “ + ” signs in the sum to “ $\times$ ” signs, what is the product then?