The problem is too difficult for us to work out.
The problem isn't that easy for us to work out.
The problem is so difficult that we can't work it out.
This is such a difficult problem that we can't work it out.
The problem is too difficult for us to work out.
The problem isn't that easy for us to work out.
The problem is so difficult that we can't work it out.
This is such a difficult problem that we can't work it out.