The mass of 4 children, Ivory, Hazel, Ivan and Pierre were taken. Only 3 children can be weighed at one time. Ivory, Hazel and Ivan weighed a total of 123 kg. Hazel, Ivan and Pierre weighed a total of 103 kg. Ivory, Ivan and Pierre weighed a total of 120 kg. Ivory, Hazel, and Pierre weighed a total of 122 kg. What is the average mass of the 4 children?
Ivory + Hazel + Ivan = 123 --- (1)
Hazel + Ivan + Pierre = 103 --- (2)
Ivory + Ivan + Pierre = 120 --- (3)
Ivory + Hazel + Pierre = 122 --- (4)

(1) + (2) + (3) + (4)
3(Ivory + Hazel + Ivan + Pierre) = 123 + 103 + 120 + 122
3(Ivory + Hazel + Ivan + Pierre) = 468
Ivory + Hazel + Ivan + Pierre = 468 ÷ 3 = 156

Total mass of the 4 children = 156

Average mass of the 4 children
= 156 ÷ 4
= 39

Answer(s): 39 kg