A container filled with 15 watermelons has a mass of 18.21 kg. The same container when filled with 8 apples has a mass of 1.41 kg. The mass of 1 watermelon is eight times the mass of 1 apple. Find the mass of the container when it is empty. Express the answer in g.
1 kg = 1000 g

18.21 kg = 18210 g
1.41 kg = 1410 g

1 apple = 1 u
8 apples = 8 x 1 u = 8 u

1 container + 8 apples = 1410
1 container + 8 u = 1410 --- (1)

1 watermelon = 8 u
15 watermelons = 15 x 8 u = 120 u

1 container + 15 watermelons = 18210
1 container + 120 u = 18210 --- (2)

(2) - (1)
120 u - 8 u = 18210 - 1410
112 u = 16800

1 u = 16800 ÷ 112 = 150

8 u = 8 x 150 = 1200

From (1):
1 container + 8 u = 1410
1 container + 1200 = 1410
 
1 container = 1410 - 1200 = 210

Mass of 1 container = 210 g

Answer(s): 210 g