The mass of 4 children, Shannon, Penelope, Billy and Valen were taken. Only 3 children can be weighed at one time. Shannon, Penelope and Billy weighed a total of 144 kg. Penelope, Billy and Valen weighed a total of 91 kg. Shannon, Billy and Valen weighed a total of 135 kg. Shannon, Penelope, and Valen weighed a total of 146 kg. What is the average mass of the 4 children?
Shannon + Penelope + Billy = 144 --- (1)
Penelope + Billy + Valen = 91 --- (2)
Shannon + Billy + Valen = 135 --- (3)
Shannon + Penelope + Valen = 146 --- (4)

(1) + (2) + (3) + (4)
3(Shannon + Penelope + Billy + Valen) = 144 + 91 + 135 + 146
3(Shannon + Penelope + Billy + Valen) = 516
Shannon + Penelope + Billy + Valen = 516 ÷ 3 = 172

Total mass of the 4 children = 172

Average mass of the 4 children
= 172 ÷ 4
= 43

Answer(s): 43 kg