perplexity; difficult question; difficult problem
面接で難問を投げかけられた。
I was posed a difficult question in the interview.
この数学の難問を解くのに何時間もかかった。
Solving this difficult math problem took hours.
環境問題は国際社会が直面している最大の難問の一つだ。
Environmental problems are one of the greatest challenges facing international society.