The mass of 4 children, Sabrina, Kathy, Justin and Vincent were taken. Only 3 children can be weighed at one time. Sabrina, Kathy and Justin weighed a total of 51 kg. Kathy, Justin and Vincent weighed a total of 133 kg. Sabrina, Justin and Vincent weighed a total of 114 kg. Sabrina, Kathy, and Vincent weighed a total of 62 kg. What is the average mass of the 4 children?
Sabrina + Kathy + Justin = 51 --- (1)
Kathy + Justin + Vincent = 133 --- (2)
Sabrina + Justin + Vincent = 114 --- (3)
Sabrina + Kathy + Vincent = 62 --- (4)

(1) + (2) + (3) + (4)
3(Sabrina + Kathy + Justin + Vincent) = 51 + 133 + 114 + 62
3(Sabrina + Kathy + Justin + Vincent) = 360
Sabrina + Kathy + Justin + Vincent = 360 ÷ 3 = 120

Total mass of the 4 children = 120

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

Answer(s): 30 kg