A container filled with 9 king melons has a mass of 9.99 kg. The same container when filled with 8 apples has a mass of 1.71 kg. The mass of 1 king melon is six times the mass of 1 apple. Find the mass of the container when it is empty. Express the answer in g.
1 kg = 1000 g

9.99 kg = 9990 g
1.71 kg = 1710 g

1 apple = 1 u
8 apples = 8 x 1 u = 8 u

1 container + 8 apples = 1710
1 container + 8 u = 1710 --- (1)

1 king melon = 6 u
9 king melons = 9 x 6 u = 54 u

1 container + 9 king melons = 9990
1 container + 54 u = 9990 --- (2)

(2) - (1)
54 u - 8 u = 9990 - 1710
46 u = 8280

1 u = 8280 ÷ 46 = 180

8 u = 8 x 180 = 1440

From (1):
1 container + 8 u = 1710
1 container + 1440 = 1710
 
1 container = 1710 - 1440 = 270

Mass of 1 container = 270 g

Answer(s): 270 g