A briefcase with 3 magazines in it has a mass of 1.5 kg. When 5 more magazines and 10 guide books are added, the mass becomes 2.5 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
1.5 kg = 1.5 x 1000 = 1500 g
2.5 kg = 2.5 x 1000 = 2500 g

Number of magazines in the end
= 3 + 5
= 8

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 + 3 M = 1500 --- (1)

1 BC + 8 M + 10 G = 2500 --- (2)

From (1)
1 BC + 9 u = 1500 --- (3)

From (2)
1 BC + 24 u + 10 u = 2500
1 BC + 34 u = 2500 --- (4)

(4) - (3)
34 u - 9 u = 2500 - 1500
25 u = 1000 

1 u = 1000 ÷ 25 = 40

From (1)
1 BC + 9 u = 1500 

From (3)
Mass of the briefcase
= 1500 - 9 u
= 1500 - (9 x 40)
= 1500 - 360
= 1140 g

1140 g = 1140 ÷ 1000 = 1.14 kg

Answer(s): 1.14 kg