A box filled with 10 watermelons has a mass of 7.5 kg. The same box when filled with 6 persimmons has a mass of 1.38 kg. The mass of 1 watermelon is four 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

7.5 kg = 7500 g
1.38 kg = 1380 g

1 persimmon = 1 u
6 persimmons = 6 x 1 u = 6 u

1 box + 6 persimmons = 1380
1 box + 6 u = 1380 --- (1)

1 watermelon = 4 u
10 watermelons = 10 x 4 u = 40 u

1 box + 10 watermelons = 7500
1 box + 40 u = 7500 --- (2)

(2) - (1)
40 u - 6 u = 7500 - 1380
34 u = 6120

1 u = 6120 ÷ 34 = 180

6 u = 6 x 180 = 1080

From (1):
1 box + 6 u = 1380
1 box + 1080 = 1380
 
1 box = 1380 - 1080 = 300

Mass of 1 box = 300 g

Answer(s): 300 g