The total weight of a toy rifle and a mini-helicopter is 3.3 kg. The total weight of a toy rifle and a toy plane is 5.9 kg. If the total weight of a toy plane and a mini-helicopter is 6.31 kg, find the total weight of the toy rifle, the mini-helicopter and the toy plane.
Let 1 toy rifle be 1 T.
Let 1 mini-helicopter be 1 M.
Let 1 toy plane be 1 P.

1 T + 1 M = 3.3 --- (1)
1 T + 1 P = 5.9 --- (2)
1 P + 1 M = 6.31 --- (3)

(1) + (2) + (3)
1 T + 1 M + 1 T + 1 P + 1 P + 1 M = 3.3 + 5.9 + 6.31
2 T + 2 M + 2 P = 15.51
2(1 T + 1 M + 1 P) = 15.51 

1 T + 1 M + 1 P = 15.51 ÷ 2
1 T + 1 M + 1 P = 7.755

 Total weight of the toy rifle, the mini-helicopter and the toy plane = 7.755 kg

Answer(s): 7.755 kg