The mass of 4 children, Fanny, Emma, Eric and Zeph were taken. Only 3 children can be weighed at one time. Fanny, Emma and Eric weighed a total of 106 kg. Emma, Eric and Zeph weighed a total of 142 kg. Fanny, Eric and Zeph weighed a total of 87 kg. Fanny, Emma, and Zeph weighed a total of 145 kg. What is the average mass of the 4 children?
Fanny + Emma + Eric = 106 --- (1)
Emma + Eric + Zeph = 142 --- (2)
Fanny + Eric + Zeph = 87 --- (3)
Fanny + Emma + Zeph = 145 --- (4)

(1) + (2) + (3) + (4)
3(Fanny + Emma + Eric + Zeph) = 106 + 142 + 87 + 145
3(Fanny + Emma + Eric + Zeph) = 480
Fanny + Emma + Eric + Zeph = 480 ÷ 3 = 160

Total mass of the 4 children = 160

Average mass of the 4 children
= 160 ÷ 4
= 40

Answer(s): 40 kg