A briefcase with 2 magazines in it has a mass of 2.108 kg. When 4 more magazines and 5 guide books are added, the mass becomes 2.924 kg. Each magazine has a mass 3 times as much as each guide book. Find the mass of the briefcase when it is empty. Express the answer in kg.
1 kg = 1000 g
2.108 kg = 2.108 x 1000 = 2108 g
2.924 kg = 2.924 x 1000 = 2924 g

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

Let 1 briefcase be 1 BC.
Let 1 magazine be 1 M.
Let 1 guide book be 1 G.

Change the magazines and the guide books into units.

1 G = 1 u
1 M = 3 u

1 BC + 2 M = 2108 --- (1)

1 BC + 6 M + 5 G = 2924 --- (2)

From (1)
1 BC + 6 u = 2108 --- (3)

From (2)
1 BC + 18 u + 5 u = 2924
1 BC + 23 u = 2924 --- (4)

(4) - (3)
23 u - 6 u = 2924 - 2108
17 u = 816 

1 u = 816 ÷ 17 = 48

From (1)
1 BC + 6 u = 2108 

From (3)
Mass of the briefcase
= 2108 - 6 u
= 2108 - (6 x 48)
= 2108 - 288
= 1820 g

1820 g = 1820 ÷ 1000 = 1.82 kg

Answer(s): 1.82 kg