Question 1175680
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Let  X  be the set of companies that produce item A;

     Y  be the set of companies that produce item B;

     Z  be the set of companies that produce item C.


You are given the numbers of elements  n(X), n(Y), n(Z),

numbers of elements in all in-pair intersections n(XY), n(XZ), n(YZ);

and the number of elements in the triple intersection n(XYZ).


They want you find n(X U Y U Z).


Use the formula


    n(X U Y U Z) = n(X) + n(Y) + n(Z) - n(XY - n(XZ) - n(YZ) + n(XYZ) = 

                 = 35 + 24 + 27 - 12 - 19 - 13 + 9 = 51.


<U>ANSWER</U>.  51 companies were contacted.
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Solved.