The number of orange files is thrice the number of blue files. The number of blue files is 5 more than the number of brown files. The total number of files is 140. The cost of each orange file is $3.30 and the cost of each brown file is $3.30. The total cost of all the files is $450.40.

1. Find the number of brown files.
2. Find the cost of a blue file.

(a)
Number of brown files = 1 u
Number of blue files = 1 u + 5

Number of orange files
= 3(1 u + 5)
= 3 u + 15

Total of number of files
= 1 u + 1 u + 5 + 3 u + 15
= 5 u + 20

5 u + 20 = 140
5 u = 140 - 20
5 u = 120

1 u = 120 ÷ 5 = 24

Number of brown files
= 1 u
= 24

(b)
Number of brown files = 24

Number of blue files
=  24 + 5
= 29

Number of orange files
= 3 x 29
= 87

Cost of brown files 
= 24 x 3.30
= $79.20

Cost of orange files 
= 87 x 3.30
= $287.10

Cost of blue files
= 450.40 - 287.10 - 79.20
= $84.10

Cost of 1 blue file
= 84.10 ÷ 29
= $2.90

Answer(s) (a) 24; (b) $2.90