The mass of 4 children, Dana, Yen, Ahmad and Billy were taken. Only 3 children can be weighed at one time. Dana, Yen and Ahmad weighed a total of 72 kg. Yen, Ahmad and Billy weighed a total of 125 kg. Dana, Ahmad and Billy weighed a total of 90 kg. Dana, Yen, and Billy weighed a total of 85 kg. What is the average mass of the 4 children?
Dana + Yen + Ahmad = 72 --- (1)
Yen + Ahmad + Billy = 125 --- (2)
Dana + Ahmad + Billy = 90 --- (3)
Dana + Yen + Billy = 85 --- (4)

(1) + (2) + (3) + (4)
3(Dana + Yen + Ahmad + Billy) = 72 + 125 + 90 + 85
3(Dana + Yen + Ahmad + Billy) = 372
Dana + Yen + Ahmad + Billy = 372 ÷ 3 = 124

Total mass of the 4 children = 124

Average mass of the 4 children
= 124 ÷ 4
= 31

Answer(s): 31 kg