A briefcase with 5 books in it has a mass of 1.68 kg. When 6 more books and 9 files are added, the mass becomes 2.598 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
1.68 kg = 1.68 x 1000 = 1680 g
2.598 kg = 2.598 x 1000 = 2598 g

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

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 + 5 B = 1680 --- (1)

1 BC + 11 B + 9 F = 2598 --- (2)

From (1)
1 BC + 15 u = 1680 --- (3)

From (2)
1 BC + 33 u + 9 u = 2598
1 BC + 42 u = 2598 --- (4)

(4) - (3)
42 u - 15 u = 2598 - 1680
27 u = 918 

1 u = 918 ÷ 27 = 34

From (1)
1 BC + 15 u = 1680 

From (3)
Mass of the briefcase
= 1680 - 15 u
= 1680 - (15 x 34)
= 1680 - 510
= 1170 g

1170 g = 1170 ÷ 1000 = 1.17 kg

Answer(s): 1.17 kg