The math problem was a hard nut to crack, but she eventually solved it
Finding the motive behind the crime proved to be a hard nut to crack for the detectives
This ancient riddle is a hard nut to crack
Negotiating a peace treaty between the two warring factions is a hard nut to crack
The company's internal security system is a hard nut to crack for hackers
Understanding his motivations has always been a hard nut to crack
This particular archaeological dig presents a hard nut to crack for researchers
Getting the landlord to agree to a rent reduction was a hard nut to crack
The final exam in quantum physics was a hard nut to crack for most students
This philosophical debate is a hard nut to crack, with no easy answers
This problem is proving difficult.
Yes, it is a hard nut to crack.
We need a new approach soon.
This math problem is a hard nut to crack
This math problem is very difficult to solve
This math problem presents a real challenge