This is the 7691st most frequent Japanese word.
単
"単" means "single" or "simple."
Here, '単' is used as a prefix in '単なる', meaning 'mere' or 'simple', to qualify the noun 'friend'.
単なる友人です。
It's just a mere friend.
In this case, '単' appears in '単に', meaning 'simply' or 'merely', to modify the manner in which the problem is being addressed.
その問題は単に数学の知識だけで解決するのは難しいです。
Solving that problem simply using mathematical knowledge alone is difficult.
Here, '単' forms part of '単純', meaning 'simple' or 'straightforward', describing the type of questions being asked.