A briefcase with 4 books in it has a mass of 2.768 kg. When 2 more books and 3 files are added, the mass becomes 3.02 kg. Each book has a mass 2 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
2.768 kg = 2.768 x 1000 = 2768 g
3.02 kg = 3.02 x 1000 = 3020 g

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

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 = 2 u

1 BC + 4 B = 2768 --- (1)

1 BC + 6 B + 3 F = 3020 --- (2)

From (1)
1 BC + 8 u = 2768 --- (3)

From (2)
1 BC + 12 u + 3 u = 3020
1 BC + 15 u = 3020 --- (4)

(4) - (3)
15 u - 8 u = 3020 - 2768
7 u = 252 

1 u = 252 ÷ 7 = 36

From (1)
1 BC + 8 u = 2768 

From (3)
Mass of the briefcase
= 2768 - 8 u
= 2768 - (8 x 36)
= 2768 - 288
= 2480 g

2480 g = 2480 ÷ 1000 = 2.48 kg

Answer(s): 2.48 kg