The answer,according to me, is A.

The question asks what is the average height of n people.

Think about averages. what is an average? It is the sum of terms/no of terms. Here it is more specific(people).

So average height of people = sum of the heights of people/ no of people or sum of heights/n.

So the question boils down to : what is n? and what is the cumulative height of people?

1) it says n/3 have 6ft 2.5 inch height and n-n/3= 2n/3 have 5ft 10 inches height.

so total height of people = n/3* 6ft 2.5 inches + 2n/3 * 5 ft 10 inches.

Avg= {n/3[ 6ft 2.5 inches+ 2* 5 ft 10 inches]}/3 n cancels out in cum and den. so you can calculate avg.

2) it gives cumulative height = 178 ft 9 inches, but ,be careful,it does not give the value of n.

You get the expression for avg -> 178ft 9 inches/n.

Not sufficient.

Hence A

what is OA?

thanks very much
the OA is A.