sandy wrote:

The average (arithmetic mean) age of the buildings on a certain city block is greater than 40 years old. If four of the buildings were built two years ago and none of the buildings are more than 80 years old, which of the following could be the number of buildings on the block?

Indicate all such numbers.

A. 4

B. 6

C. 8

D. 11

E. 40

let the number of buildings = n

number of buildings built 2yrs ago = 4 ;total age in yrs of 4 buildings = 2*4 = 8

remaining buildings = n-4 ; total age in years of (n-4) buildings = 80(n-4)

number of buildings = 8 + 80(n-4)/n >40

when n=4, 8<40

when n=6, 28<40

when n=8, 41>40

when n=11, 51.64>40

when n=40, 72.2>40

hence, ans is C,D,E