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    Ok i have some questions like the one below but im not sure what the method is for solving them, i've had a go at doing it, I think are probably other way than using sets to solve this type of question but i need to do it using sets.

    Of 46 malicious hackers, each of whom writes at least one of boot sector, para-
    sitic and macro viruses (see
    for explanations) 6 only write boot sector viruses, 9 only write parasitic viruses,
    and 27 will write macro viruses. How many will write boot sector and parasitic,
    but not macro viruses?

    My attempt

    A = the set of boot sector viruses
    B = the set of parastic viruses
    C = the set of macro viruses

    |A union B union C| = 46

    |A union B union C| - |A| - |B| - |C| + |A union B|
    46 - 6 - 9 - 27 + |A union B|
    46 - 6 - 9 - 27 + = 4

    So i have 4 left over but how do i now work out :-
    |A union B| and |A union C| |B union C| as im only interested in finding
    |A union B|
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