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.
No one could solve this difficult math problem.
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.