difficult problem

That said, the U.S. intelligence community assesses that ISIS poses a particularly difficult problem.

In all her bold, dashing, adventurous life she had never been confronted by a more difficult problem than this.

The girl who presents the most difficult problem in club life is the one whose social impulses are dominant.

I should call it a most extraordinary approach toward the settlement of a difficult problem.

"We have discussed rather a difficult problem and not settled it," he says in a conventional tone.

The more difficult problem was how any predicates at all can be applied to God without endangering his unity.