Question 942477
If three years ago the average was 18, today the average is 21 so
(A+B)/2=21 
A+B=42
with C the average is 22 so
(A+B+C)/3=22
A+B+C=66 Substitute A+B from above
(A+B)+C=66
42+C=66 subtract 42 from each side
C=24 so C is currently 24 years old