A box filled with 9 king melons has a mass of 7.83 kg. The same box when filled with 3 passion fruits has a mass of 0.69 kg. The mass of 1 king melon is five 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

7.83 kg = 7830 g
0.69 kg = 690 g

1 passion fruit = 1 u
3 passion fruits = 3 x 1 u = 3 u

1 box + 3 passion fruits = 690
1 box + 3 u = 690 --- (1)

1 king melon = 5 u
9 king melons = 9 x 5 u = 45 u

1 box + 9 king melons = 7830
1 box + 45 u = 7830 --- (2)

(2) - (1)
45 u - 3 u = 7830 - 690
42 u = 7140

1 u = 7140 ÷ 42 = 170

3 u = 3 x 170 = 510

From (1):
1 box + 3 u = 690
1 box + 510 = 690
 
1 box = 690 - 510 = 180

Mass of 1 box = 180 g

Answer(s): 180 g