1.   Calculations for transition metal species present particularly difficult problems.

2.   During the eighteenth and nineteenth centuries many studies were undertaken in hopes of solving the difficult problems of animal locomotion.

3.   For many young people this is the most difficult problem that they have ever had to deal with in their lives.

4.   Hers is an honest response to a difficult problem.

5.   Making contact with the families in the first instance was the most difficult problem.

6.   Much of the joy typical of Great Groups seems to reflect the profound pleasure humans take in solving difficult problems.

7.   Naturally, difficult problems will remain in various countries where the faction fight has been long and bitter.

8.   Particularly during successful long-term participant observation, the borderline between overt and covert recording can become blurred and quite difficult problems emerge.

9.   Since he was elected, the president has been faced with a host of difficult problems.