She had known Max for some years now, but he was still pretty much an unknown quantity.

她现在认识马克斯也好几年了，但是他仍然是个大大的未知数。

Are an unknown quantity.

对于他们来说是一个未知的组合。

In algebra, the sign X usually denotes an unknown quantity.

在代数里，符号X常用来表示未知数。

John is a possibility for the job, but he is still an unknown quantity.

约翰可能获得这工作但现在还是个未知数。

Mary is a possibility for the position, but she is still an unknown quantity.

玛丽可能获得这工作，但(能否获得)现在还是个未知数。