The mass of 4 children, Cathy, Betty, Paul and Vaidev were taken. Only 3 children can be weighed at one time. Cathy, Betty and Paul weighed a total of 136 kg. Betty, Paul and Vaidev weighed a total of 71 kg. Cathy, Paul and Vaidev weighed a total of 147 kg. Cathy, Betty, and Vaidev weighed a total of 114 kg. What is the average mass of the 4 children?
Cathy + Betty + Paul = 136 --- (1)
Betty + Paul + Vaidev = 71 --- (2)
Cathy + Paul + Vaidev = 147 --- (3)
Cathy + Betty + Vaidev = 114 --- (4)

(1) + (2) + (3) + (4)
3(Cathy + Betty + Paul + Vaidev) = 136 + 71 + 147 + 114
3(Cathy + Betty + Paul + Vaidev) = 468
Cathy + Betty + Paul + Vaidev = 468 ÷ 3 = 156

Total mass of the 4 children = 156

Average mass of the 4 children
= 156 ÷ 4
= 39

Answer(s): 39 kg