A box filled with 9 watermelons has a mass of 5.95 kg. The same box when filled with 4 mangosteens has a mass of 0.83 kg. The mass of 1 watermelon is four times the mass of 1 mangosteen. Find the mass of the box when it is empty. Express the answer in g.
1 kg = 1000 g

5.95 kg = 5950 g
0.83 kg = 830 g

1 mangosteen = 1 u
4 mangosteens = 4 x 1 u = 4 u

1 box + 4 mangosteens = 830
1 box + 4 u = 830 --- (1)

1 watermelon = 4 u
9 watermelons = 9 x 4 u = 36 u

1 box + 9 watermelons = 5950
1 box + 36 u = 5950 --- (2)

(2) - (1)
36 u - 4 u = 5950 - 830
32 u = 5120

1 u = 5120 ÷ 32 = 160

4 u = 4 x 160 = 640

From (1):
1 box + 4 u = 830
1 box + 640 = 830
 
1 box = 830 - 640 = 190

Mass of 1 box = 190 g

Answer(s): 190 g