The mass of 4 children, Marion, Barbara, Will and Rael were taken. Only 3 children can be weighed at one time. Marion, Barbara and Will weighed a total of 129 kg. Barbara, Will and Rael weighed a total of 105 kg. Marion, Will and Rael weighed a total of 122 kg. Marion, Barbara, and Rael weighed a total of 88 kg. What is the average mass of the 4 children?
Marion + Barbara + Will = 129 --- (1)
Barbara + Will + Rael = 105 --- (2)
Marion + Will + Rael = 122 --- (3)
Marion + Barbara + Rael = 88 --- (4)

(1) + (2) + (3) + (4)
3(Marion + Barbara + Will + Rael) = 129 + 105 + 122 + 88
3(Marion + Barbara + Will + Rael) = 444
Marion + Barbara + Will + Rael = 444 ÷ 3 = 148

Total mass of the 4 children = 148

Average mass of the 4 children
= 148 ÷ 4
= 37

Answer(s): 37 kg