prep-2-133 DS 已知数未知数的疑问

卮言浅夏

what is the average (arithmetic mean) height of the n people in a certain group?
(1) the average height of the n/3 tallest people n the group is 6 feet 2.5inches ,and the average height of the rest of the people n the group is f feet 10 inches
(2) the sum of the heights of the n people is 178 feet 9 inches.

选A ，所以说A项中的f是算作已知数吗
这种已知未知怎样判断呀  谢谢

silence05

我一直不懂这道题是个什么问题 不应该选D么
如果说N算作已知的话