The total weight of a flying saucer and a water gun is 2.9 kg. The total weight of a flying saucer and a toy train is 5.1 kg. If the total weight of a toy train and a water gun is 5.59 kg, find the total weight of the flying saucer, the water gun and the toy train.
Let 1 flying saucer be 1 F.
Let 1 water gun be 1 W.
Let 1 toy train be 1 T.

1 F + 1 W = 2.9 --- (1)
1 F + 1 T = 5.1 --- (2)
1 T + 1 W = 5.59 --- (3)

(1) + (2) + (3)
1 F + 1 W + 1 F + 1 T + 1 T + 1 W = 2.9 + 5.1 + 5.59
2 F + 2 W + 2 T = 13.59
2(1 F + 1 W + 1 T) = 13.59 

1 F + 1 W + 1 T = 13.59 ÷ 2
1 F + 1 W + 1 T = 6.795

 Total weight of the flying saucer, the water gun and the toy train = 6.795 kg

Answer(s): 6.795 kg