arithmetic - five years ago the average age of a and b was 15 years average age of a b and c today is 20 years how old will c be after 10 years - skillgun

(A + B), five years ago = (15 * 2) = 30 years.
(A+B), now = (30+5*2)years=40 years.
(A + B + C), now = (20 x 3) years =60 years.
C, now = (60 - 40) years = 20 years.
C, after 10 years = (20 + 10) years =30 years.