A full solution can be found in the link I gave in the comments, but since this is a rather tricky problem, let me give step-by-step hints in case you want to work out the details yourself.
1) Let $n$ be the number of questions. Set up a table with $n$ rows and $441=21^2$ columns, where each row corresponds to a question and each column to a (boy, girl) pair. Let an entry be $1$ if both members of the pair solve the question, otherwise $0$.
2) You know that for every (boy, girl) pair, there is at least one question that they both solve. What does this tell you about the number of $1$'s in the table?
3) You want to show that some row (question) is solved by at least three boys and three girls. So assume for contradiction that each row is solved by at most two boys or two girls.
4) Fix a girl $g$. All columns that correspond to her can have a $1$ in at most six rows. There are at least $21$ $1$'s in these columns and the six rows. If a row (question) is solved by at most two boys, there are at most two $1$'s in the row. Out of the (at most) six rows, at most how many rows solved by at most two boys can there be? At most how many $1$'s are there in total among these rows?
5) Summing up over all girls $g$, at most how many $1$'s are there in rows solved by at most two boys? By symmetry, there are at most the same number of $1$'s in rows solved by at most two girls. But these two classes together include all rows, by our assumption for contradiction.
6) Derive a contradiction between 2) and 5).