The problem is too difficult for us to work out.(改为同义句)

The problem is 【so 】difficult that 【none of us 】 can work it out.
too...for sb.to do 太...以至于不能...
= so.that sb.can't do sth.如此...以至于某人不能...
The problem is () difficult that () can work it out.此句中由于 can 是肯定式,因此只能用主语来表示否定.
故用【none of us 】.如果只能填一个词的话,就填【nobody】 即:
The problem is 【so 】difficult that 【nobody】 can work it out.