30. A round cake is cut into $n$ pieces with 3 cuts. Find the product of all possible values of $n$.
30. Answer: 840 .
With three cuts, a round cake can be cut into at least $1+3=4$ pieces, and at most $1+1+2+3=7$ pieces. Moreover, $n=4,5,6,7$ are all possible. $4 \times 5 \times 6 \times 7=840$.