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。

《柯林斯英汉双解大词典》

Are an unknown quantity.

对于他们来说是一个未知的组合。

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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。

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But in some key repertoire for the Philharmonic, Bruckner and Schumann for instance, he is an unknown quantity.

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At 18, Prince Hamzeh is an unknown quantity (just as King Hussein was when he became king, aged 17).

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"He's seen as an unknown quantity, and VCs love for things to be very much in control, " says Facebook cofounder Dustin Moskovitz.

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