The mass of 4 children, Xandra, Emma, Brandon and Albert were taken. Only 3 children can be weighed at one time. Xandra, Emma and Brandon weighed a total of 122 kg. Emma, Brandon and Albert weighed a total of 149 kg. Xandra, Brandon and Albert weighed a total of 76 kg. Xandra, Emma, and Albert weighed a total of 61 kg. What is the average mass of the 4 children?
Xandra + Emma + Brandon = 122 --- (1)
Emma + Brandon + Albert = 149 --- (2)
Xandra + Brandon + Albert = 76 --- (3)
Xandra + Emma + Albert = 61 --- (4)

(1) + (2) + (3) + (4)
3(Xandra + Emma + Brandon + Albert) = 122 + 149 + 76 + 61
3(Xandra + Emma + Brandon + Albert) = 408
Xandra + Emma + Brandon + Albert = 408 ÷ 3 = 136

Total mass of the 4 children = 136

Average mass of the 4 children
= 136 ÷ 4
= 34

Answer(s): 34 kg