A briefcase with 2 magazines in it has a mass of 2.374 kg. When 5 more magazines and 11 guide books are added, the mass becomes 3.34 kg. Each magazine has a mass 2 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
2.374 kg = 2.374 x 1000 = 2374 g
3.34 kg = 3.34 x 1000 = 3340 g

Number of magazines in the end
= 2 + 5
= 7

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 = 2 u

1 BC + 2 M = 2374 --- (1)

1 BC + 7 M + 11 G = 3340 --- (2)

From (1)
1 BC + 4 u = 2374 --- (3)

From (2)
1 BC + 14 u + 11 u = 3340
1 BC + 25 u = 3340 --- (4)

(4) - (3)
25 u - 4 u = 3340 - 2374
21 u = 966 

1 u = 966 ÷ 21 = 46

From (1)
1 BC + 4 u = 2374 

From (3)
Mass of the briefcase
= 2374 - 4 u
= 2374 - (4 x 46)
= 2374 - 184
= 2190 g

2190 g = 2190 ÷ 1000 = 2.19 kg

Answer(s): 2.19 kg