A briefcase with 3 books in it has a mass of 1.49 kg. When 4 more books and 5 files are added, the mass becomes 2.17 kg. Each book has a mass 3 times as much as each file. Find the mass of the briefcase when it is empty. Express the answer in kg.
1 kg = 1000 g
1.49 kg = 1.49 x 1000 = 1490 g
2.17 kg = 2.17 x 1000 = 2170 g

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

Let 1 briefcase be 1 BC.
Let 1 book be 1 B.
Let 1 file be 1 F.

Change the books and the files into units.

1 F = 1 u
1 B = 3 u

1 BC + 3 B = 1490 --- (1)

1 BC + 7 B + 5 F = 2170 --- (2)

From (1)
1 BC + 9 u = 1490 --- (3)

From (2)
1 BC + 21 u + 5 u = 2170
1 BC + 26 u = 2170 --- (4)

(4) - (3)
26 u - 9 u = 2170 - 1490
17 u = 680 

1 u = 680 ÷ 17 = 40

From (1)
1 BC + 9 u = 1490 

From (3)
Mass of the briefcase
= 1490 - 9 u
= 1490 - (9 x 40)
= 1490 - 360
= 1130 g

1130 g = 1130 ÷ 1000 = 1.13 kg

Answer(s): 1.13 kg