The problem is very difficult.We can't work it out.(51.合并）
1.The problem is ---- ---- --- to work out.(difficult for us)
2.The problem isn't ---- ---- ---- --- to work out.(easy enough for us)
3.The problem is ---- ---- --- we can't work it out.(so difficult that)
4.This is --- a difficult problem--- we can't work it out.(such,that)