1)We know two averages, one of the first 1/3 of the group and the other of the 2/3 of the group. SO we can write it like that:
Average = ((n/3)*6ft 2.5in + (2n/3)*5ft 10in ) / n
You can put n on factor and then
Average = (6ft 2.5in)/3 + (2*5ft 10in )/3
So you have your average. Sorry I do not calculate it coz I am not in a feet/inch country.

2) We do not know n, it could be 10 or 20 and it would change the average height.

(A)

GMAC has the knack to make the problems look horrendous !

(1) Average height of (n/3) tallest people in the group is 6ft 2.5in and the average height of the rest of the people in the group is 5ft 10in.

n/3 means so there are three person and one of them is blabla inches

so blabla x 1 + other bla bla x2 / 3 = AV

No need to get lost with inches

(2) Sum of the heights of the n people is 178 ft 9 inches.

we now the sum but we dont now the N

Insuf

Typical C trap.

for quick calculation.

if we have a set A and two sub set B and C such that.. Combined B and C make A and there is no item common b/w B and C then.

If we have saperate AVG of B and C then we can get the avg of A.

Ans : A.