The number of green files is thrice the number of blue files. The number of blue files is 8 more than the number of yellow files. The total number of files is 137. The cost of each green file is $3.60 and the cost of each yellow file is $2.50. The total cost of all the files is $432.40.

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

(a)
Number of yellow files = 1 u
Number of blue files = 1 u + 8

Number of green files
= 3(1 u + 8)
= 3 u + 24

Total of number of files
= 1 u + 1 u + 8 + 3 u + 24
= 5 u + 32

5 u + 32 = 137
5 u = 137 - 32
5 u = 105

1 u = 105 ÷ 5 = 21

Number of yellow files
= 1 u
= 21

(b)
Number of yellow files = 21

Number of blue files
=  21 + 8
= 29

Number of green files
= 3 x 29
= 87

Cost of yellow files 
= 21 x 2.50
= $52.50

Cost of green files 
= 87 x 3.60
= $313.20

Cost of blue files
= 432.40 - 313.20 - 52.50
= $66.70

Cost of 1 blue file
= 66.70 ÷ 29
= $2.30

Answer(s) (a) 21; (b) $2.30