A briefcase with 4 books in it has a mass of 2.292 kg. When 3 more books and 12 magazines are added, the mass becomes 2.724 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
2.292 kg = 2.292 x 1000 = 2292 g
2.724 kg = 2.724 x 1000 = 2724 g

Number of books in the end
= 4 + 3
= 7

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 + 4 B = 2292 --- (1)

1 BC + 7 B + 12 M = 2724 --- (2)

From (1)
1 BC + 8 u = 2292 --- (3)

From (2)
1 BC + 14 u + 12 u = 2724
1 BC + 26 u = 2724 --- (4)

(4) - (3)
26 u - 8 u = 2724 - 2292
18 u = 432 

1 u = 432 ÷ 18 = 24

From (1)
1 BC + 8 u = 2292 

From (3)
Mass of the briefcase
= 2292 - 8 u
= 2292 - (8 x 24)
= 2292 - 192
= 2100 g

2100 g = 2100 ÷ 1000 = 2.1 kg

Answer(s): 2.1 kg