A briefcase with 4 magazines in it has a mass of 1.632 kg. When 3 more magazines and 9 guide books are added, the mass becomes 2.28 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
1.632 kg = 1.632 x 1000 = 1632 g
2.28 kg = 2.28 x 1000 = 2280 g

Number of magazines in the end
= 4 + 3
= 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 = 3 u

1 BC + 4 M = 1632 --- (1)

1 BC + 7 M + 9 G = 2280 --- (2)

From (1)
1 BC + 12 u = 1632 --- (3)

From (2)
1 BC + 21 u + 9 u = 2280
1 BC + 30 u = 2280 --- (4)

(4) - (3)
30 u - 12 u = 2280 - 1632
18 u = 648 

1 u = 648 ÷ 18 = 36

From (1)
1 BC + 12 u = 1632 

From (3)
Mass of the briefcase
= 1632 - 12 u
= 1632 - (12 x 36)
= 1632 - 432
= 1200 g

1200 g = 1200 ÷ 1000 = 1.2 kg

Answer(s): 1.2 kg