Earlier quoted context omitted.
>"Based on the national estimate, the majority of concussions resulted from participation in football (40.5%, n = 55 007), followed by girls' soccer (21.5%, n = 29 167), boys' soccer (15.4%, n = 20 929),..." >So 40.5% of concussions in football and 36.9% in combined boys/girls soccer. If 21.5% of girls (~6270) and 15.4% of boys (~3223) get concussions then 9493 (6270 + 3223) received concussions out of 50096 (20929 +…
Unfortunately n, in this paper, doesn't seem to mean sample size like you would expect in statistics. It is an extrapolated number of concussions nationwide based on a study of 100 high schools.
Or, if you can figure out the relative difference between the number of boys and girls who play soccer, you can do a weighted average of the percentages.
Either way, adding the percentages is completely incorrect.
EDIT: http://www.usyouthsoccer.org/media_kit/keystatistics/ suggests that the split is roughly equal -- in 2008 it was 52% boys and 48% girls.
With that said, the correct weighted value is more like 21.5x.48 + 15.4x.52 = 18.3%, which is significantly less than the 40.5% you quoted for football.