A box filled with 9 watermelons has a mass of 10.98 kg. The same box when filled with 9 oranges has a mass of 1.53 kg. The mass of 1 watermelon is eight times the mass of 1 orange. Find the mass of the box when it is empty. Express the answer in g.
1 kg = 1000 g

10.98 kg = 10980 g
1.53 kg = 1530 g

1 orange = 1 u
9 oranges = 9 x 1 u = 9 u

1 box + 9 oranges = 1530
1 box + 9 u = 1530 --- (1)

1 watermelon = 8 u
9 watermelons = 9 x 8 u = 72 u

1 box + 9 watermelons = 10980
1 box + 72 u = 10980 --- (2)

(2) - (1)
72 u - 9 u = 10980 - 1530
63 u = 9450

1 u = 9450 ÷ 63 = 150

9 u = 9 x 150 = 1350

From (1):
1 box + 9 u = 1530
1 box + 1350 = 1530
 
1 box = 1530 - 1350 = 180

Mass of 1 box = 180 g

Answer(s): 180 g