Now you're shown us line 9 of the pattern I'm even more convinced that 'S' is a mis-print for '5'. Just like on line 10 you have 13 sitiches before the pattern repeat (between the asterisks) and 3 stitches after the pattern repeat. If you subsitute 5 for S in each line you have 25 stitches in both pattern repeats, which go exactly into the 100 remaining stitches your mother will be knitting.
An English Knitter (nationality and method).
FO: Downton Hat
Call me either Clutter or Julie (my real name), if you find it easier to type than Dclutterchique.