The mass of 4 children, Risa, Marion, Eric and Will were taken. Only 3 children can be weighed at one time. Risa, Marion and Eric weighed a total of 87 kg. Marion, Eric and Will weighed a total of 50 kg. Risa, Eric and Will weighed a total of 100 kg. Risa, Marion, and Will weighed a total of 51 kg. What is the average mass of the 4 children?
Risa + Marion + Eric = 87 --- (1)
Marion + Eric + Will = 50 --- (2)
Risa + Eric + Will = 100 --- (3)
Risa + Marion + Will = 51 --- (4)

(1) + (2) + (3) + (4)
3(Risa + Marion + Eric + Will) = 87 + 50 + 100 + 51
3(Risa + Marion + Eric + Will) = 288
Risa + Marion + Eric + Will = 288 ÷ 3 = 96

Total mass of the 4 children = 96

Average mass of the 4 children
= 96 ÷ 4
= 24

Answer(s): 24 kg