The mass of 4 children, Hilda, Anna, Neave and Fabian were taken. Only 3 children can be weighed at one time. Hilda, Anna and Neave weighed a total of 78 kg. Anna, Neave and Fabian weighed a total of 61 kg. Hilda, Neave and Fabian weighed a total of 149 kg. Hilda, Anna, and Fabian weighed a total of 72 kg. What is the average mass of the 4 children?
Hilda + Anna + Neave = 78 --- (1)
Anna + Neave + Fabian = 61 --- (2)
Hilda + Neave + Fabian = 149 --- (3)
Hilda + Anna + Fabian = 72 --- (4)

(1) + (2) + (3) + (4)
3(Hilda + Anna + Neave + Fabian) = 78 + 61 + 149 + 72
3(Hilda + Anna + Neave + Fabian) = 360
Hilda + Anna + Neave + Fabian = 360 ÷ 3 = 120

Total mass of the 4 children = 120

Average mass of the 4 children
= 120 ÷ 4
= 30

Answer(s): 30 kg