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。