A box filled with 13 mangoes has a mass of 13.18 kg. The same box when filled with 5 oranges has a mass of 1.18 kg. The mass of 1 mango is five times the mass of 1 orange. Find the mass of the box when it is empty. Express the answer in g.
1 kg = 1000 g

13.18 kg = 13180 g
1.18 kg = 1180 g

1 orange = 1 u
5 oranges = 5 x 1 u = 5 u

1 box + 5 oranges = 1180
1 box + 5 u = 1180 --- (1)

1 mango = 5 u
13 mangoes = 13 x 5 u = 65 u

1 box + 13 mangoes = 13180
1 box + 65 u = 13180 --- (2)

(2) - (1)
65 u - 5 u = 13180 - 1180
60 u = 12000

1 u = 12000 ÷ 60 = 200

5 u = 5 x 200 = 1000

From (1):
1 box + 5 u = 1180
1 box + 1000 = 1180
 
1 box = 1180 - 1000 = 180

Mass of 1 box = 180 g

Answer(s): 180 g