A briefcase with 2 books in it has a mass of 2.736 kg. When 4 more books and 6 magazines are added, the mass becomes 3.564 kg. Each book has a mass 3 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
2.736 kg = 2.736 x 1000 = 2736 g
3.564 kg = 3.564 x 1000 = 3564 g

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

Let 1 briefcase be 1 BC.
Let 1 book be 1 B.
Let 1 magazine be 1 M.

Change the books and the magazines into units.

1 M = 1 u
1 B = 3 u

1 BC + 2 B = 2736 --- (1)

1 BC + 6 B + 6 M = 3564 --- (2)

From (1)
1 BC + 6 u = 2736 --- (3)

From (2)
1 BC + 18 u + 6 u = 3564
1 BC + 24 u = 3564 --- (4)

(4) - (3)
24 u - 6 u = 3564 - 2736
18 u = 828 

1 u = 828 ÷ 18 = 46

From (1)
1 BC + 6 u = 2736 

From (3)
Mass of the briefcase
= 2736 - 6 u
= 2736 - (6 x 46)
= 2736 - 276
= 2460 g

2460 g = 2460 ÷ 1000 = 2.46 kg

Answer(s): 2.46 kg