The letter "x" often represents an unknown number in some Math exercises.

在一些数学练习中，字母“x”通常代表一个未知数。

youdao

A couple of hours after I left the comment, I got a text message from an unknown number.

我评价过后几个小时，收到了一个陌生号码的信息。

youdao

Officials say an unknown number of militants in hostages were killed in the ensuing gun battle.

官员表示人质群里数量不明的武装分子在随后的枪战中丧生。

youdao

Diakite,who is also known as Toumba, escaped the attack and is still at large with an unknown number of soldiers from the presidential guard.

VOA: standard.2009.12.07

About 31 Japanese companies had offices in the buildings employing an unknown number of staff.

CNN: Asia pledges cooperation in hunt for attackers

There could be an unknown number of other sports pitches that have been lost, it said.

BBC: Olympics legacy: How can we encourage kids into sport?

That unknown number quite possibly being higher than the jobs created that we can see.

FORBES: If Apple Onshored iPad Production it Would Create 67,000 American Manufacturing Jobs!