A box filled with 13 watermelons has a mass of 12.66 kg. The same box when filled with 9 pears has a mass of 1.26 kg. The mass of 1 watermelon is eight 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

12.66 kg = 12660 g
1.26 kg = 1260 g

1 pear = 1 u
9 pears = 9 x 1 u = 9 u

1 box + 9 pears = 1260
1 box + 9 u = 1260 --- (1)

1 watermelon = 8 u
13 watermelons = 13 x 8 u = 104 u

1 box + 13 watermelons = 12660
1 box + 104 u = 12660 --- (2)

(2) - (1)
104 u - 9 u = 12660 - 1260
95 u = 11400

1 u = 11400 ÷ 95 = 120

9 u = 9 x 120 = 1080

From (1):
1 box + 9 u = 1260
1 box + 1080 = 1260
 
1 box = 1260 - 1080 = 180

Mass of 1 box = 180 g

Answer(s): 180 g