I’m betting this is made flat. If so the stitch on each edge is going to be used to sew the hat up. So to get rid of 10 the first time you decrease you want to k the edge st, then (k5, k2tog) repeat across to the last st, knit 1. That would make 10 decreases evenly spaced and the edge st on each side.
I think you count 3 and decrease on the 4th row starting to count after row 16. I don’t know why they wrote it that way, but if you started decreasing on the 17th row you would be through with the 5 rows of decreases before you reached row 36. Maybe they change the stitch after the 16 rows of garter, but you didn’t say so.
After the first decrease row knit 3 rows with no changes. The next row you will decrease. K1 edge st, (k4, k2tog) repeat across to the last st, k1.
Do the 3 rows and decrease again. Each time you decrease another row do the edge stitches and do one fewer stitch between the decreases. Keep doing that until you have decreased 5 times and have 22 sts left. You have decreased away 50 sts.