The mass of 4 children, Olivia, Penelope, Valen and Harry were taken. Only 3 children can be weighed at one time. Olivia, Penelope and Valen weighed a total of 124 kg. Penelope, Valen and Harry weighed a total of 113 kg. Olivia, Valen and Harry weighed a total of 135 kg. Olivia, Penelope, and Harry weighed a total of 96 kg. What is the average mass of the 4 children?
Olivia + Penelope + Valen = 124 --- (1)
Penelope + Valen + Harry = 113 --- (2)
Olivia + Valen + Harry = 135 --- (3)
Olivia + Penelope + Harry = 96 --- (4)

(1) + (2) + (3) + (4)
3(Olivia + Penelope + Valen + Harry) = 124 + 113 + 135 + 96
3(Olivia + Penelope + Valen + Harry) = 468
Olivia + Penelope + Valen + Harry = 468 ÷ 3 = 156

Total mass of the 4 children = 156

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

Answer(s): 39 kg