Question 1010632
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10 + 10 - 15 = 20 - 15 = 5.


For details, see the lesson <A HREF=http://www.algebra.com/algebra/homework/word/misc/Counting-elements-in-sub-sets-of-a-given-finite-set.lesson>Counting elements in sub-sets of a given finite set</A> in this site.