The mass of 4 children, Shiyun, Zara, Zane and Ken were taken. Only 3 children can be weighed at one time. Shiyun, Zara and Zane weighed a total of 78 kg. Zara, Zane and Ken weighed a total of 117 kg. Shiyun, Zane and Ken weighed a total of 90 kg. Shiyun, Zara, and Ken weighed a total of 51 kg. What is the average mass of the 4 children?
Shiyun + Zara + Zane = 78 --- (1)
Zara + Zane + Ken = 117 --- (2)
Shiyun + Zane + Ken = 90 --- (3)
Shiyun + Zara + Ken = 51 --- (4)

(1) + (2) + (3) + (4)
3(Shiyun + Zara + Zane + Ken) = 78 + 117 + 90 + 51
3(Shiyun + Zara + Zane + Ken) = 336
Shiyun + Zara + Zane + Ken = 336 ÷ 3 = 112

Total mass of the 4 children = 112

Average mass of the 4 children
= 112 ÷ 4
= 28

Answer(s): 28 kg