The average age of the members of a 10-person committee is the same as it was 4 years ago, because an old member has just been replaced by a new member.
How much younger is the new member than the old member who was replaced?
Let the sum of nine member (total) =x
and the age of old one=z
so its average 4 yrs before=(x+z)/10.
after 4 yrs let z be replaced by y.
so now avg=(x+4*10+y)/10
so after solving it found
so the older person is 40yrs older than young one
:) (sorry for the typo)