How to improve the quality of products was a difficult problem to him.

　　如何提高產品質量對他來說可是個難題。

　　She is working at a difficult problem in mathematics.

　　她在做一道數學難題。

　　How to grow rice in a dry area is a difficult problem.

　　在干旱地區種水稻可是一個難題。

　　When you come across a difficult problem, think about it by yourself first.

　　遇到難題的時候先自己想想。

　　The student decided to work out the difficult problem by himself.

　　這個學生決心獨自解決這道難題。

　　Air pollution is a difficult problem for environmental protect.

　　城市大氣污染問題是環境保護的一項難題。