A container filled with 15 jackfruits has a mass of 7.5 kg. The same container when filled with 4 mangosteens has a mass of 0.78 kg. The mass of 1 jackfruit is four times the mass of 1 mangosteen. Find the mass of the container when it is empty. Express the answer in g.
1 kg = 1000 g

7.5 kg = 7500 g
0.78 kg = 780 g

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

1 container + 4 mangosteens = 780
1 container + 4 u = 780 --- (1)

1 jackfruit = 4 u
15 jackfruits = 15 x 4 u = 60 u

1 container + 15 jackfruits = 7500
1 container + 60 u = 7500 --- (2)

(2) - (1)
60 u - 4 u = 7500 - 780
56 u = 6720

1 u = 6720 ÷ 56 = 120

4 u = 4 x 120 = 480

From (1):
1 container + 4 u = 780
1 container + 480 = 780
 
1 container = 780 - 480 = 300

Mass of 1 container = 300 g

Answer(s): 300 g