A briefcase with 3 files in it has a mass of 1.702 kg. When 5 more files and 7 magazines are added, the mass becomes 2.318 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
1.702 kg = 1.702 x 1000 = 1702 g
2.318 kg = 2.318 x 1000 = 2318 g

Number of files in the end
= 3 + 5
= 8

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 + 3 F = 1702 --- (1)

1 BC + 8 F + 7 M = 2318 --- (2)

From (1)
1 BC + 9 u = 1702 --- (3)

From (2)
1 BC + 24 u + 7 u = 2318
1 BC + 31 u = 2318 --- (4)

(4) - (3)
31 u - 9 u = 2318 - 1702
22 u = 616 

1 u = 616 ÷ 22 = 28

From (1)
1 BC + 9 u = 1702 

From (3)
Mass of the briefcase
= 1702 - 9 u
= 1702 - (9 x 28)
= 1702 - 252
= 1450 g

1450 g = 1450 ÷ 1000 = 1.45 kg

Answer(s): 1.45 kg