A box filled with 15 king melons has a mass of 13.94 kg. The same box when filled with 7 chikoos has a mass of 1.2 kg. The mass of 1 king melon is seven times the mass of 1 chikoo. Find the mass of the box when it is empty. Express the answer in g.
1 kg = 1000 g

13.94 kg = 13940 g
1.2 kg = 1200 g

1 chikoo = 1 u
7 chikoos = 7 x 1 u = 7 u

1 box + 7 chikoos = 1200
1 box + 7 u = 1200 --- (1)

1 king melon = 7 u
15 king melons = 15 x 7 u = 105 u

1 box + 15 king melons = 13940
1 box + 105 u = 13940 --- (2)

(2) - (1)
105 u - 7 u = 13940 - 1200
98 u = 12740

1 u = 12740 ÷ 98 = 130

7 u = 7 x 130 = 910

From (1):
1 box + 7 u = 1200
1 box + 910 = 1200
 
1 box = 1200 - 910 = 290

Mass of 1 box = 290 g

Answer(s): 290 g