The mass of 4 children, Roshel, Nicole, Glen and Gabriel were taken. Only 3 children can be weighed at one time. Roshel, Nicole and Glen weighed a total of 78 kg. Nicole, Glen and Gabriel weighed a total of 72 kg. Roshel, Glen and Gabriel weighed a total of 72 kg. Roshel, Nicole, and Gabriel weighed a total of 138 kg. What is the average mass of the 4 children?
Roshel + Nicole + Glen = 78 --- (1)
Nicole + Glen + Gabriel = 72 --- (2)
Roshel + Glen + Gabriel = 72 --- (3)
Roshel + Nicole + Gabriel = 138 --- (4)

(1) + (2) + (3) + (4)
3(Roshel + Nicole + Glen + Gabriel) = 78 + 72 + 72 + 138
3(Roshel + Nicole + Glen + Gabriel) = 360
Roshel + Nicole + Glen + Gabriel = 360 ÷ 3 = 120

Total mass of the 4 children = 120

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

Answer(s): 30 kg