A box filled with 9 honeydews has a mass of 11.61 kg. The same box when filled with 3 starfruits has a mass of 0.81 kg. The mass of 1 honeydew is seven times the mass of 1 starfruit. Find the mass of the box when it is empty. Express the answer in g.
1 kg = 1000 g

11.61 kg = 11610 g
0.81 kg = 810 g

1 starfruit = 1 u
3 starfruits = 3 x 1 u = 3 u

1 box + 3 starfruits = 810
1 box + 3 u = 810 --- (1)

1 honeydew = 7 u
9 honeydews = 9 x 7 u = 63 u

1 box + 9 honeydews = 11610
1 box + 63 u = 11610 --- (2)

(2) - (1)
63 u - 3 u = 11610 - 810
60 u = 10800

1 u = 10800 ÷ 60 = 180

3 u = 3 x 180 = 540

From (1):
1 box + 3 u = 810
1 box + 540 = 810
 
1 box = 810 - 540 = 270

Mass of 1 box = 270 g

Answer(s): 270 g