A box filled with 10 king melons has a mass of 8.61 kg. The same box when filled with 7 oranges has a mass of 1.19 kg. The mass of 1 king melon is six 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

8.61 kg = 8610 g
1.19 kg = 1190 g

1 orange = 1 u
7 oranges = 7 x 1 u = 7 u

1 box + 7 oranges = 1190
1 box + 7 u = 1190 --- (1)

1 king melon = 6 u
10 king melons = 10 x 6 u = 60 u

1 box + 10 king melons = 8610
1 box + 60 u = 8610 --- (2)

(2) - (1)
60 u - 7 u = 8610 - 1190
53 u = 7420

1 u = 7420 ÷ 53 = 140

7 u = 7 x 140 = 980

From (1):
1 box + 7 u = 1190
1 box + 980 = 1190
 
1 box = 1190 - 980 = 210

Mass of 1 box = 210 g

Answer(s): 210 g