The mass of 4 children, Elyse, Eva, Albert and Ian were taken. Only 3 children can be weighed at one time. Elyse, Eva and Albert weighed a total of 51 kg. Eva, Albert and Ian weighed a total of 67 kg. Elyse, Albert and Ian weighed a total of 64 kg. Elyse, Eva, and Ian weighed a total of 58 kg. What is the average mass of the 4 children?
Elyse + Eva + Albert = 51 --- (1)
Eva + Albert + Ian = 67 --- (2)
Elyse + Albert + Ian = 64 --- (3)
Elyse + Eva + Ian = 58 --- (4)

(1) + (2) + (3) + (4)
3(Elyse + Eva + Albert + Ian) = 51 + 67 + 64 + 58
3(Elyse + Eva + Albert + Ian) = 240
Elyse + Eva + Albert + Ian = 240 ÷ 3 = 80

Total mass of the 4 children = 80

Average mass of the 4 children
= 80 ÷ 4
= 20

Answer(s): 20 kg