The three unknowns can be specified as follows:

Let x = the number of stocks of OHaganBooks.com you purchased.
Let y = the number of stocks of FarmersBooks.com you purchased.
Let z = the number of stocks of JungleBooks.com you purchased.

You can also list these in some other order, for instance:

Let x = the number of stocks of FarmersBooks.com you purchased.
Let y = the number of stocks of JungleBooks.com you purchased.
Let z = the number of stocks of OHaganBooks.com you purchased.

NoteFor the rest of the problem, we shall assume the first ordering.

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