A briefcase with 2 files in it has a mass of 2.206 kg. When 4 more files and 5 magazines are added, the mass becomes 2.648 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
2.206 kg = 2.206 x 1000 = 2206 g
2.648 kg = 2.648 x 1000 = 2648 g

Number of files in the end
= 2 + 4
= 6

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 + 2 F = 2206 --- (1)

1 BC + 6 F + 5 M = 2648 --- (2)

From (1)
1 BC + 4 u = 2206 --- (3)

From (2)
1 BC + 12 u + 5 u = 2648
1 BC + 17 u = 2648 --- (4)

(4) - (3)
17 u - 4 u = 2648 - 2206
13 u = 442 

1 u = 442 ÷ 13 = 34

From (1)
1 BC + 4 u = 2206 

From (3)
Mass of the briefcase
= 2206 - 4 u
= 2206 - (4 x 34)
= 2206 - 136
= 2070 g

2070 g = 2070 ÷ 1000 = 2.07 kg

Answer(s): 2.07 kg