Can anyone help me understand what this part of a knitting pattern is asking me to do?

Question

Continue in Rib 1. At the same time, when sleeve measures 18 cm, increase 2 sts mid under arm (on either side of a purled st) on every 3.5 cm a total of 10 times = 56-60-64 sts. Incorporate the increased sts in Rib1 as you go along.

Answer 1

I'm guessing the rib 1 is the stitch pattern?  It means to work the sleeve in the established stitch until it reaches a length of 18 cm.  After that, increase two stitches somewhere in the middle of the underarm (apparently there are purl stitches, put the increases either before or after the purl stitch).  What I get from it is that you increase every 3.5 cm, a total of 10 times and you should have the amount of stitches when you are done that it shows.

Answer 2

Work your pattern even for 18cm. Then, figure out where the middle of the underarm of the sleeve would be. There will be a purl st there. You will be making 1 increase on each side of the center purl st on this row, and then work the regular pattern for 3.5 cm, working these 2 increases in the pattern; then repeat with the increases and working 3.5 cm as before, for a total of 10 increase rows. the 56-60-64 sts is your ending total of sts on your needle.