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

But for people who have a difficult time controlling their movements, this little navigational issue can be a really big problem.

VOA: special.2011.04.18

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

The most difficult national problem facing the administration of President Franklin Pierce was the situation in Kansas.

VOA: special.2009.05.07

"The problem with that body of research is that it's difficult to interpret because obviously people with existing mental health problems use cigarettes to self-medicate."

VOA: standard.2010.06.15