difficult problem; challenge
Refers to a tough issue, puzzle, or task that is hard to solve or overcome. Often used in academic, business, or personal contexts.
この数学の難題は誰にも解けなかった。
No one could solve this difficult math problem.
彼はいつも難題に挑戦するのが好きだ。
He always likes to take on challenges.
The boss confronted his subordinate with an unreasonable demand.
The company is saddled with the difficult problem of a lack of funds.
The researchers have been tackling that difficult problem for years.
He brilliantly solved the difficult problem.