The mass of 4 children, Barbara, Hilda, Zeph and Luke were taken. Only 3 children can be weighed at one time. Barbara, Hilda and Zeph weighed a total of 122 kg. Hilda, Zeph and Luke weighed a total of 52 kg. Barbara, Zeph and Luke weighed a total of 99 kg. Barbara, Hilda, and Luke weighed a total of 123 kg. What is the average mass of the 4 children?
Barbara + Hilda + Zeph = 122 --- (1)
Hilda + Zeph + Luke = 52 --- (2)
Barbara + Zeph + Luke = 99 --- (3)
Barbara + Hilda + Luke = 123 --- (4)

(1) + (2) + (3) + (4)
3(Barbara + Hilda + Zeph + Luke) = 122 + 52 + 99 + 123
3(Barbara + Hilda + Zeph + Luke) = 396
Barbara + Hilda + Zeph + Luke = 396 ÷ 3 = 132

Total mass of the 4 children = 132

Average mass of the 4 children
= 132 ÷ 4
= 33

Answer(s): 33 kg