A briefcase with 2 files in it has a mass of 2.618 kg. When 3 more files and 7 magazines are added, the mass becomes 3.034 kg. Each file has a mass 2 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
2.618 kg = 2.618 x 1000 = 2618 g
3.034 kg = 3.034 x 1000 = 3034 g

Number of files in the end
= 2 + 3
= 5

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 = 2 u

1 BC + 2 F = 2618 --- (1)

1 BC + 5 F + 7 M = 3034 --- (2)

From (1)
1 BC + 4 u = 2618 --- (3)

From (2)
1 BC + 10 u + 7 u = 3034
1 BC + 17 u = 3034 --- (4)

(4) - (3)
17 u - 4 u = 3034 - 2618
13 u = 416 

1 u = 416 ÷ 13 = 32

From (1)
1 BC + 4 u = 2618 

From (3)
Mass of the briefcase
= 2618 - 4 u
= 2618 - (4 x 32)
= 2618 - 128
= 2490 g

2490 g = 2490 ÷ 1000 = 2.49 kg

Answer(s): 2.49 kg