What comes next in the series 8, 4, 12, 6, 18, 9, ?

Explanation

1. 01

   The terms go down, up, down, up, so look at the operations: $8 \div 2 = 4$, $4 \times 3 = 12$, $12 \div 2 = 6$, $6 \times 3 = 18$, $18 \div 2 = 9$.

2. 02

   The operations alternate $\div 2$ and $\times 3$. The next is $\times 3$: $9 \times 3 = 27$.