求助~~PREP 133

哪位高人能帮我解一下这道题。谢谢了

133. 14229-!-item-!-187;#058&010329 答案是A

What is the average (arithmetic mean) height of the n people in a certain group?

(1) The average height of the 3/n tallest people n the group is 6 feet 2*1/2(2又1/2) inches, 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.