A briefcase with 3 books in it has a mass of 1.496 kg. When 2 more books and 10 magazines are added, the mass becomes 2.04 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
1.496 kg = 1.496 x 1000 = 1496 g
2.04 kg = 2.04 x 1000 = 2040 g

Number of books in the end
= 3 + 2
= 5

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 + 3 B = 1496 --- (1)

1 BC + 5 B + 10 M = 2040 --- (2)

From (1)
1 BC + 9 u = 1496 --- (3)

From (2)
1 BC + 15 u + 10 u = 2040
1 BC + 25 u = 2040 --- (4)

(4) - (3)
25 u - 9 u = 2040 - 1496
16 u = 544 

1 u = 544 ÷ 16 = 34

From (1)
1 BC + 9 u = 1496 

From (3)
Mass of the briefcase
= 1496 - 9 u
= 1496 - (9 x 34)
= 1496 - 306
= 1190 g

1190 g = 1190 ÷ 1000 = 1.19 kg

Answer(s): 1.19 kg