A briefcase with 3 books in it has a mass of 2.316 kg. When 6 more books and 12 files are added, the mass becomes 3.336 kg. Each book has a mass 3 times as much as each file. Find the mass of the briefcase when it is empty. Express the answer in kg.
1 kg = 1000 g
2.316 kg = 2.316 x 1000 = 2316 g
3.336 kg = 3.336 x 1000 = 3336 g

Number of books in the end
= 3 + 6
= 9

Let 1 briefcase be 1 BC.
Let 1 book be 1 B.
Let 1 file be 1 F.

Change the books and the files into units.

1 F = 1 u
1 B = 3 u

1 BC + 3 B = 2316 --- (1)

1 BC + 9 B + 12 F = 3336 --- (2)

From (1)
1 BC + 9 u = 2316 --- (3)

From (2)
1 BC + 27 u + 12 u = 3336
1 BC + 39 u = 3336 --- (4)

(4) - (3)
39 u - 9 u = 3336 - 2316
30 u = 1020 

1 u = 1020 ÷ 30 = 34

From (1)
1 BC + 9 u = 2316 

From (3)
Mass of the briefcase
= 2316 - 9 u
= 2316 - (9 x 34)
= 2316 - 306
= 2010 g

2010 g = 2010 ÷ 1000 = 2.01 kg

Answer(s): 2.01 kg