You do a decrease at each end of rows 10, 20 & 30. So if this is the RS, you ssk at start & k2tog at the end.
If the chart isn't marked for the decreases, remember to compensate by ignoring the first stitch each side rows 11-20. The first & last 2 stitches, rows 21-30. And the first & last 3 stitches, rows 31 on.