A container filled with 10 watermelons has a mass of 7.21 kg. The same container when filled with 4 oranges has a mass of 0.77 kg. The mass of 1 watermelon is five times the mass of 1 orange. Find the mass of the container when it is empty. Express the answer in g.
1 kg = 1000 g

7.21 kg = 7210 g
0.77 kg = 770 g

1 orange = 1 u
4 oranges = 4 x 1 u = 4 u

1 container + 4 oranges = 770
1 container + 4 u = 770 --- (1)

1 watermelon = 5 u
10 watermelons = 10 x 5 u = 50 u

1 container + 10 watermelons = 7210
1 container + 50 u = 7210 --- (2)

(2) - (1)
50 u - 4 u = 7210 - 770
46 u = 6440

1 u = 6440 ÷ 46 = 140

4 u = 4 x 140 = 560

From (1):
1 container + 4 u = 770
1 container + 560 = 770
 
1 container = 770 - 560 = 210

Mass of 1 container = 210 g

Answer(s): 210 g