That unsolved problem keeps him awake at night.

That unsolved case keeps him awake at night.

I am very interested in social science.

He thinks that war is a necessary evil.

Do you understand the necessity of this project?

That information is unnecessary.

That information is completely unnecessary.

We crossed the border and entered a new country.

A new approach is needed to solve this set of problems.

The main problem with this project is the lack of communication.