A briefcase with 5 books in it has a mass of 1.96 kg. When 6 more books and 9 magazines are added, the mass becomes 2.716 kg. Each book has a mass 2 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
1.96 kg = 1.96 x 1000 = 1960 g
2.716 kg = 2.716 x 1000 = 2716 g

Number of books in the end
= 5 + 6
= 11

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 = 2 u

1 BC + 5 B = 1960 --- (1)

1 BC + 11 B + 9 M = 2716 --- (2)

From (1)
1 BC + 10 u = 1960 --- (3)

From (2)
1 BC + 22 u + 9 u = 2716
1 BC + 31 u = 2716 --- (4)

(4) - (3)
31 u - 10 u = 2716 - 1960
21 u = 756 

1 u = 756 ÷ 21 = 36

From (1)
1 BC + 10 u = 1960 

From (3)
Mass of the briefcase
= 1960 - 10 u
= 1960 - (10 x 36)
= 1960 - 360
= 1600 g

1600 g = 1600 ÷ 1000 = 1.6 kg

Answer(s): 1.6 kg