The mass of 4 children, Jen, Anna, Xavier and Albert were taken. Only 3 children can be weighed at one time. Jen, Anna and Xavier weighed a total of 121 kg. Anna, Xavier and Albert weighed a total of 52 kg. Jen, Xavier and Albert weighed a total of 107 kg. Jen, Anna, and Albert weighed a total of 56 kg. What is the average mass of the 4 children?
Jen + Anna + Xavier = 121 --- (1)
Anna + Xavier + Albert = 52 --- (2)
Jen + Xavier + Albert = 107 --- (3)
Jen + Anna + Albert = 56 --- (4)

(1) + (2) + (3) + (4)
3(Jen + Anna + Xavier + Albert) = 121 + 52 + 107 + 56
3(Jen + Anna + Xavier + Albert) = 336
Jen + Anna + Xavier + Albert = 336 ÷ 3 = 112

Total mass of the 4 children = 112

Average mass of the 4 children
= 112 ÷ 4
= 28

Answer(s): 28 kg