急问一道prep数学题。 两天就考了

prep 题目里出现的x, n之类的要当作未知数还是 已知数呢？

比如：

A certain group of car dealerships agreed to donate x dollars to Red Cross Chapter for each car sold during a 30-day period. what was the total amount that was expected to be donated. ?

1) a total of 500 cars were expected to be sold.

2) 60 more cars were sold than expected, so that the total amount actually donated was $28,000.

难道此处x不应该是已知数么？答案是c, 我选的是D。

还有一道说有n个人，问是否求出n个人的平均身高

1）前三分之一平均是高为多少多少，后三分之2平均是高为多少多少

2）n个人的总身高是多少。

答案是a,我选的d， 看来是把n当成未知数了？？