The mass of 4 children, Fanny, Anna, Wesley and Valen were taken. Only 3 children can be weighed at one time. Fanny, Anna and Wesley weighed a total of 77 kg. Anna, Wesley and Valen weighed a total of 56 kg. Fanny, Wesley and Valen weighed a total of 45 kg. Fanny, Anna, and Valen weighed a total of 62 kg. What is the average mass of the 4 children?
Fanny + Anna + Wesley = 77 --- (1)
Anna + Wesley + Valen = 56 --- (2)
Fanny + Wesley + Valen = 45 --- (3)
Fanny + Anna + Valen = 62 --- (4)

(1) + (2) + (3) + (4)
3(Fanny + Anna + Wesley + Valen) = 77 + 56 + 45 + 62
3(Fanny + Anna + Wesley + Valen) = 240
Fanny + Anna + Wesley + Valen = 240 ÷ 3 = 80

Total mass of the 4 children = 80

Average mass of the 4 children
= 80 ÷ 4
= 20

Answer(s): 20 kg