The way I'm reading it is:
Knit 8 rows, and on the 9th row you'll decrease 3 times, once at the beginning, once in the center and once at the end of the row.
Then you continue knitting even in garter stitch, and on row 11, decrease 1 at the beginning, 2 in the center and 1 at the end. Knit row 12, and then repeat row 11.
The math works out, that's decreasing 11 stitches. What isn't clear though is that it only actually tells you to knit 9 rows, not 13. So are you supposed to reach 50 stitches by row 9, or are you knitting more than 9 rows... <shrug>
Just from what you posted, the above is what I would do.