Determination of which positions are likely to be disease associated is a challenging and important problem.

He came to the job facing a challenging problem.

One of the most challenging problems for companies, worldwide, is pilfering.

This is a challenging problem, because social systems are self-referential: the act of learning changes the thing to be learned.

This is a challenging problem, but perhaps it's not insurmountable.

Data was pleased to have such a challenging problem to work on.

The most challenging problems facing all of education are in the middle schools.

Whether they are working for love or money, scientists say hair is a challenging problem.

"From the discussions I have, it is certainly the most challenging problem at the moment," he said.

It suggests that challenging problems lie ahead for any buyer.