A box filled with 11 mangoes has a mass of 10.71 kg. The same box when filled with 5 persimmons has a mass of 1.21 kg. The mass of 1 mango is five times the mass of 1 persimmon. Find the mass of the box when it is empty. Express the answer in g.
1 kg = 1000 g

10.71 kg = 10710 g
1.21 kg = 1210 g

1 persimmon = 1 u
5 persimmons = 5 x 1 u = 5 u

1 box + 5 persimmons = 1210
1 box + 5 u = 1210 --- (1)

1 mango = 5 u
11 mangoes = 11 x 5 u = 55 u

1 box + 11 mangoes = 10710
1 box + 55 u = 10710 --- (2)

(2) - (1)
55 u - 5 u = 10710 - 1210
50 u = 9500

1 u = 9500 ÷ 50 = 190

5 u = 5 x 190 = 950

From (1):
1 box + 5 u = 1210
1 box + 950 = 1210
 
1 box = 1210 - 950 = 260

Mass of 1 box = 260 g

Answer(s): 260 g