The mass of 4 children, Gwen, Tiffany, Rael and Ahmad were taken. Only 3 children can be weighed at one time. Gwen, Tiffany and Rael weighed a total of 45 kg. Tiffany, Rael and Ahmad weighed a total of 64 kg. Gwen, Rael and Ahmad weighed a total of 119 kg. Gwen, Tiffany, and Ahmad weighed a total of 84 kg. What is the average mass of the 4 children?
Gwen + Tiffany + Rael = 45 --- (1)
Tiffany + Rael + Ahmad = 64 --- (2)
Gwen + Rael + Ahmad = 119 --- (3)
Gwen + Tiffany + Ahmad = 84 --- (4)

(1) + (2) + (3) + (4)
3(Gwen + Tiffany + Rael + Ahmad) = 45 + 64 + 119 + 84
3(Gwen + Tiffany + Rael + Ahmad) = 312
Gwen + Tiffany + Rael + Ahmad = 312 ÷ 3 = 104

Total mass of the 4 children = 104

Average mass of the 4 children
= 104 ÷ 4
= 26

Answer(s): 26 kg