[求助]一道PREP数学题

For all positive integers m, [m]=3m when m is odd and [m]=0.5m when m is even. Which of the following is equivalent to [9]x[6]?

81
54
36
27
18

答案选27。但我算出的结果是81。9是奇数，故[9]=3x9=27。而6是偶数，故[6]=0.5x6=3。所以[9]x[6]=27x3=81。请问我的计算过程错在哪里呢？

Thanks!

maybe u could think about doing that way

9*6 =54

0.5*54=27<--the answer