A box filled with 8 mangoes has a mass of 7.81 kg. The same box when filled with 5 persimmons has a mass of 1.16 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

7.81 kg = 7810 g
1.16 kg = 1160 g

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

1 box + 5 persimmons = 1160
1 box + 5 u = 1160 --- (1)

1 mango = 5 u
8 mangoes = 8 x 5 u = 40 u

1 box + 8 mangoes = 7810
1 box + 40 u = 7810 --- (2)

(2) - (1)
40 u - 5 u = 7810 - 1160
35 u = 6650

1 u = 6650 ÷ 35 = 190

5 u = 5 x 190 = 950

From (1):
1 box + 5 u = 1160
1 box + 950 = 1160
 
1 box = 1160 - 950 = 210

Mass of 1 box = 210 g

Answer(s): 210 g