This is the 10249th most frequent Japanese word.
難問
Difficult problem.
難問 is used to describe a specific math problem as challenging and requiring extra effort.
この数学の問題は難問です。
This math problem is a difficult question.
難問 refers to a challenging problem that one is actively trying to solve.
彼はその難問に挑戦しました。
He tackled that difficult question.
難問 is used metaphorically here to describe complex and challenging tasks or situations faced by a company.