Today,the solution to the problem is so common that it is difficult to understand that there was a problem at all.

VOA: special.2010.07.14

She began to explain to the students how to solve a very difficult problem.

VOA: special.2009.10.25

She was never willing to leave a problem unfinished, even difficult problems in mathematics.

VOA: special.2009.11.08

Hobrooke admits is a very complicated and difficult problem to address.

VOA: standard.2009.06.15