A briefcase with 5 files in it has a mass of 2.14 kg. When 6 more files and 9 magazines are added, the mass becomes 3.022 kg. Each file has a mass 2 times as much as each magazine. Find the mass of the briefcase when it is empty. Express the answer in kg.
1 kg = 1000 g
2.14 kg = 2.14 x 1000 = 2140 g
3.022 kg = 3.022 x 1000 = 3022 g

Number of files in the end
= 5 + 6
= 11

Let 1 briefcase be 1 BC.
Let 1 file be 1 F.
Let 1 magazine be 1 M.

Change the files and the magazines into units.

1 M = 1 u
1 F = 2 u

1 BC + 5 F = 2140 --- (1)

1 BC + 11 F + 9 M = 3022 --- (2)

From (1)
1 BC + 10 u = 2140 --- (3)

From (2)
1 BC + 22 u + 9 u = 3022
1 BC + 31 u = 3022 --- (4)

(4) - (3)
31 u - 10 u = 3022 - 2140
21 u = 882 

1 u = 882 ÷ 21 = 42

From (1)
1 BC + 10 u = 2140 

From (3)
Mass of the briefcase
= 2140 - 10 u
= 2140 - (10 x 42)
= 2140 - 420
= 1720 g

1720 g = 1720 ÷ 1000 = 1.72 kg

Answer(s): 1.72 kg