A briefcase with 4 files in it has a mass of 1.694 kg. When 2 more files and 3 magazines are added, the mass becomes 1.982 kg. Each file has a mass 3 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
1.694 kg = 1.694 x 1000 = 1694 g
1.982 kg = 1.982 x 1000 = 1982 g

Number of files in the end
= 4 + 2
= 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 = 3 u

1 BC + 4 F = 1694 --- (1)

1 BC + 6 F + 3 M = 1982 --- (2)

From (1)
1 BC + 12 u = 1694 --- (3)

From (2)
1 BC + 18 u + 3 u = 1982
1 BC + 21 u = 1982 --- (4)

(4) - (3)
21 u - 12 u = 1982 - 1694
9 u = 288 

1 u = 288 ÷ 9 = 32

From (1)
1 BC + 12 u = 1694 

From (3)
Mass of the briefcase
= 1694 - 12 u
= 1694 - (12 x 32)
= 1694 - 384
= 1310 g

1310 g = 1310 ÷ 1000 = 1.31 kg

Answer(s): 1.31 kg