A box filled with 14 watermelons has a mass of 20.32 kg. The same box when filled with 7 persimmons has a mass of 1.42 kg. The mass of 1 watermelon is eight times the mass of 1 persimmon. Find the mass of the box when it is empty. Express the answer in g.
1 kg = 1000 g

20.32 kg = 20320 g
1.42 kg = 1420 g

1 persimmon = 1 u
7 persimmons = 7 x 1 u = 7 u

1 box + 7 persimmons = 1420
1 box + 7 u = 1420 --- (1)

1 watermelon = 8 u
14 watermelons = 14 x 8 u = 112 u

1 box + 14 watermelons = 20320
1 box + 112 u = 20320 --- (2)

(2) - (1)
112 u - 7 u = 20320 - 1420
105 u = 18900

1 u = 18900 ÷ 105 = 180

7 u = 7 x 180 = 1260

From (1):
1 box + 7 u = 1420
1 box + 1260 = 1420
 
1 box = 1420 - 1260 = 160

Mass of 1 box = 160 g

Answer(s): 160 g