The mass of 4 children, Wendy, Gabby, Nick and David were taken. Only 3 children can be weighed at one time. Wendy, Gabby and Nick weighed a total of 108 kg. Gabby, Nick and David weighed a total of 48 kg. Wendy, Nick and David weighed a total of 117 kg. Wendy, Gabby, and David weighed a total of 87 kg. What is the average mass of the 4 children?
Wendy + Gabby + Nick = 108 --- (1)
Gabby + Nick + David = 48 --- (2)
Wendy + Nick + David = 117 --- (3)
Wendy + Gabby + David = 87 --- (4)

(1) + (2) + (3) + (4)
3(Wendy + Gabby + Nick + David) = 108 + 48 + 117 + 87
3(Wendy + Gabby + Nick + David) = 360
Wendy + Gabby + Nick + David = 360 ÷ 3 = 120

Total mass of the 4 children = 120

Average mass of the 4 children
= 120 ÷ 4
= 30

Answer(s): 30 kg