Question 1129419
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The set  (A' &#8745; B)  is the set of those elements of B, that do not belong to A.


So  (A' &#8745; B)  is simply the difference of two subsets  B  and  (A &#8745; B) :


    (A' &#8745; B) = B \ (A &#8745; B),      (*)


where the symbol  " \ "  symbolizes subtraction of subsets.


From (*),  n(A' &#8745; B) = n(B) - n(A &#8745; B) = 60 - 10 = 50.    <U>ANSWER</U>
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Solved.