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a) Let F be the cost of a file.
Let B be the cost of a book.

2B + F = 11.20 …(1)

B = 3F …(2)

Putting (2) into (1), we get
2(3F) + F = 11.20
6F + F = 11.20
7F = 11.20
F = $1.60 #

b) B = 3F = 3(1.60) = 4.80

Let m be the number of books marcus bought.
Let n be the number of files marcus bought.

So,
m(4.80) + n(1.60) = 192 …(1)

m – n = 8
m(4.80) – n(4.80) = 8(4.80) = 38.40 …(2)

Subtracting (2) from (1):
n(1.60) + n(4.80) = 192 – 38.40 = 153.60
n(6.40) = 153.60
n = 24 #

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(a)  If a file costs $u, then a book costs $3u.

2* 3u + u = 11.2

7u = 11.2

u = 1.6

A file costs $1.6.

(b) 8 books costs  $ 1.6 * 3 * 8 = $ 38.4 

A book and a file costs $ 1.6 *4 = $ 6.4

So number of files bought = (192 – 38.4) / 6.4 = 24

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