A briefcase with 5 books in it has a mass of 2.49 kg. When 4 more books and 12 files are added, the mass becomes 3.29 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
2.49 kg = 2.49 x 1000 = 2490 g
3.29 kg = 3.29 x 1000 = 3290 g

Number of books in the end
= 5 + 4
= 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 = 2 u

1 BC + 5 B = 2490 --- (1)

1 BC + 9 B + 12 F = 3290 --- (2)

From (1)
1 BC + 10 u = 2490 --- (3)

From (2)
1 BC + 18 u + 12 u = 3290
1 BC + 30 u = 3290 --- (4)

(4) - (3)
30 u - 10 u = 3290 - 2490
20 u = 800 

1 u = 800 ÷ 20 = 40

From (1)
1 BC + 10 u = 2490 

From (3)
Mass of the briefcase
= 2490 - 10 u
= 2490 - (10 x 40)
= 2490 - 400
= 2090 g

2090 g = 2090 ÷ 1000 = 2.09 kg

Answer(s): 2.09 kg