The total weight of a toy mask and a mini-helicopter is 2.9 kg. The total weight of a toy mask and a toy car is 4.9 kg. If the total weight of a toy car and a mini-helicopter is 5.12 kg, find the total weight of the toy mask, the mini-helicopter and the toy car.
Let 1 toy mask be 1 T.
Let 1 mini-helicopter be 1 M.
Let 1 toy car be 1 C.

1 T + 1 M = 2.9 --- (1)
1 T + 1 C = 4.9 --- (2)
1 C + 1 M = 5.12 --- (3)

(1) + (2) + (3)
1 T + 1 M + 1 T + 1 C + 1 C + 1 M = 2.9 + 4.9 + 5.12
2 T + 2 M + 2 C = 12.92
2(1 T + 1 M + 1 C) = 12.92 

1 T + 1 M + 1 C = 12.92 ÷ 2
1 T + 1 M + 1 C = 6.46

 Total weight of the toy mask, the mini-helicopter and the toy car = 6.46 kg

Answer(s): 6.46 kg