难题 (nán tí): ปัญหาที่ยาก; ปัญหาที่ท้าทาย; คำถามที่ท้าทาย

This problem is very hard; it is a real difficult problem.

We need to solve this technical challenge.

When you encounter a difficult problem, don't give up easily.

The teacher gave the students a difficult math problem.