Event "A" is defined as "soccer players who are younger than 9 years old"
Looking at the table, there are two categories that fit the definition of the event, those are the categories for players with ages 7 and 8.
The complement of A will represent all other players, this means, the soccer players who are 9 years old or older. The categories that fit this description are those for players with ages 9 and 10.
To determine the number of players that correspond to the complement of A, you have to add the number of players that are 9 years old and the number of players that are 10 years old:
There are 10 players in the complement of A.