Answer:
a) 490
b) 240
Explanation:
b) It is convenient to answer part B first.
The number owning only a fax will be the number owning a fax less the number that own both a fax and a copier:
420 -180 = 240 . . . . number owning only a fax
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a) The number owning one or the other will be the number owing a copier (which includes those owning both) plus the number owning only a fax:
250 +240 = 490 . . . . number owning either
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If you want to work part A first, then you need to use the relation ...
n(A∪B) = n(A) +n(B) -n(A∩B)