A box filled with 13 king melons has a mass of 15.88 kg. The same box when filled with 4 pears has a mass of 1.08 kg. The mass of 1 king melon is six times the mass of 1 pear. Find the mass of the box when it is empty. Express the answer in g.
1 kg = 1000 g

15.88 kg = 15880 g
1.08 kg = 1080 g

1 pear = 1 u
4 pears = 4 x 1 u = 4 u

1 box + 4 pears = 1080
1 box + 4 u = 1080 --- (1)

1 king melon = 6 u
13 king melons = 13 x 6 u = 78 u

1 box + 13 king melons = 15880
1 box + 78 u = 15880 --- (2)

(2) - (1)
78 u - 4 u = 15880 - 1080
74 u = 14800

1 u = 14800 ÷ 74 = 200

4 u = 4 x 200 = 800

From (1):
1 box + 4 u = 1080
1 box + 800 = 1080
 
1 box = 1080 - 800 = 280

Mass of 1 box = 280 g

Answer(s): 280 g