A briefcase with 2 books in it has a mass of 1.302 kg. When 3 more books and 11 magazines are added, the mass becomes 1.742 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
1.302 kg = 1.302 x 1000 = 1302 g
1.742 kg = 1.742 x 1000 = 1742 g

Number of books in the end
= 2 + 3
= 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 + 2 B = 1302 --- (1)

1 BC + 5 B + 11 M = 1742 --- (2)

From (1)
1 BC + 6 u = 1302 --- (3)

From (2)
1 BC + 15 u + 11 u = 1742
1 BC + 26 u = 1742 --- (4)

(4) - (3)
26 u - 6 u = 1742 - 1302
20 u = 440 

1 u = 440 ÷ 20 = 22

From (1)
1 BC + 6 u = 1302 

From (3)
Mass of the briefcase
= 1302 - 6 u
= 1302 - (6 x 22)
= 1302 - 132
= 1170 g

1170 g = 1170 ÷ 1000 = 1.17 kg

Answer(s): 1.17 kg