The mass of 4 children, Irene, Jaslyn, David and Mark were taken. Only 3 children can be weighed at one time. Irene, Jaslyn and David weighed a total of 126 kg. Jaslyn, David and Mark weighed a total of 97 kg. Irene, David and Mark weighed a total of 64 kg. Irene, Jaslyn, and Mark weighed a total of 109 kg. What is the average mass of the 4 children?
Irene + Jaslyn + David = 126 --- (1)
Jaslyn + David + Mark = 97 --- (2)
Irene + David + Mark = 64 --- (3)
Irene + Jaslyn + Mark = 109 --- (4)

(1) + (2) + (3) + (4)
3(Irene + Jaslyn + David + Mark) = 126 + 97 + 64 + 109
3(Irene + Jaslyn + David + Mark) = 396
Irene + Jaslyn + David + Mark = 396 ÷ 3 = 132

Total mass of the 4 children = 132

Average mass of the 4 children
= 132 ÷ 4
= 33

Answer(s): 33 kg