A box filled with 9 watermelons has a mass of 9.33 kg. The same box when filled with 7 chikoos has a mass of 1.34 kg. The mass of 1 watermelon is six times the mass of 1 chikoo. Find the mass of the box when it is empty. Express the answer in g.
1 kg = 1000 g

9.33 kg = 9330 g
1.34 kg = 1340 g

1 chikoo = 1 u
7 chikoos = 7 x 1 u = 7 u

1 box + 7 chikoos = 1340
1 box + 7 u = 1340 --- (1)

1 watermelon = 6 u
9 watermelons = 9 x 6 u = 54 u

1 box + 9 watermelons = 9330
1 box + 54 u = 9330 --- (2)

(2) - (1)
54 u - 7 u = 9330 - 1340
47 u = 7990

1 u = 7990 ÷ 47 = 170

7 u = 7 x 170 = 1190

From (1):
1 box + 7 u = 1340
1 box + 1190 = 1340
 
1 box = 1340 - 1190 = 150

Mass of 1 box = 150 g

Answer(s): 150 g