The mass of 4 children, Wendy, Tammy, Ivan and Andy were taken. Only 3 children can be weighed at one time. Wendy, Tammy and Ivan weighed a total of 149 kg. Tammy, Ivan and Andy weighed a total of 102 kg. Wendy, Ivan and Andy weighed a total of 68 kg. Wendy, Tammy, and Andy weighed a total of 137 kg. What is the average mass of the 4 children?
Wendy + Tammy + Ivan = 149 --- (1)
Tammy + Ivan + Andy = 102 --- (2)
Wendy + Ivan + Andy = 68 --- (3)
Wendy + Tammy + Andy = 137 --- (4)

(1) + (2) + (3) + (4)
3(Wendy + Tammy + Ivan + Andy) = 149 + 102 + 68 + 137
3(Wendy + Tammy + Ivan + Andy) = 456
Wendy + Tammy + Ivan + Andy = 456 ÷ 3 = 152

Total mass of the 4 children = 152

Average mass of the 4 children
= 152 ÷ 4
= 38

Answer(s): 38 kg