Answer:
Math:The Union of A and B is the set of all those elements which belong either to A or to B or both A and B. use the notation A U B (which is read as 'A union B') to denote the union of set A and set B.
Algebra: n(A U B) = n(A) + n(B) - n(A n B) so 20 = n(A) + 10 - 5 OR n(A) = 15.
I found these as a match for what they stand for, not sure if any of them are exactly what you need, but i hope this helps ^_^
