The average age of Helen, Sharen, Claire and Donna is 20 years. Sharen is 8 years older than Helen and 15 years older than Claire. The sum of the ages of Helen and Sharen is 46, while that of Sharen and Claire is 39.
What is Donna's age?
If we name each chca by its initial, from the statement we obtain the following equations:
(H + S + C + D) / 4 = 20
S = H + 8
S = C + 15
H + S = 46
S + C = 39
If we solve the system we get that Donna is 22 years old.