since this is a DS qn, you dont even have to worry abt inches..just work with feet and see if you can get one single value with out any variable.
1) The average height of the n/3 tallest people in the group is 6 feet 2 1/2 inches, and the average height of the rest of the people in the group is 5 feet 10 inches.
A = S /N, S = AN
S= 2N && S= 10/3N
N you get one value hence A.
B is tempting but you can leave N there. A= 178/N. going no where with out N.