The mass of 4 children, Anna, Jane, Eric and Ahmad were taken. Only 3 children can be weighed at one time. Anna, Jane and Eric weighed a total of 142 kg. Jane, Eric and Ahmad weighed a total of 58 kg. Anna, Eric and Ahmad weighed a total of 96 kg. Anna, Jane, and Ahmad weighed a total of 148 kg. What is the average mass of the 4 children?
Anna + Jane + Eric = 142 --- (1)
Jane + Eric + Ahmad = 58 --- (2)
Anna + Eric + Ahmad = 96 --- (3)
Anna + Jane + Ahmad = 148 --- (4)

(1) + (2) + (3) + (4)
3(Anna + Jane + Eric + Ahmad) = 142 + 58 + 96 + 148
3(Anna + Jane + Eric + Ahmad) = 444
Anna + Jane + Eric + Ahmad = 444 ÷ 3 = 148

Total mass of the 4 children = 148

Average mass of the 4 children
= 148 ÷ 4
= 37

Answer(s): 37 kg