A box filled with 9 durians has a mass of 7.2 kg. The same box when filled with 4 passion fruits has a mass of 0.71 kg. The mass of 1 durian is seven times the mass of 1 passion fruit. Find the mass of the box when it is empty. Express the answer in g.
1 kg = 1000 g

7.2 kg = 7200 g
0.71 kg = 710 g

1 passion fruit = 1 u
4 passion fruits = 4 x 1 u = 4 u

1 box + 4 passion fruits = 710
1 box + 4 u = 710 --- (1)

1 durian = 7 u
9 durians = 9 x 7 u = 63 u

1 box + 9 durians = 7200
1 box + 63 u = 7200 --- (2)

(2) - (1)
63 u - 4 u = 7200 - 710
59 u = 6490

1 u = 6490 ÷ 59 = 110

4 u = 4 x 110 = 440

From (1):
1 box + 4 u = 710
1 box + 440 = 710
 
1 box = 710 - 440 = 270

Mass of 1 box = 270 g

Answer(s): 270 g