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Sets 1.6

Problem

X and Y are two sets such that X has 15 elements, Y has 15 elements and XY has 20 elements.
Find the number of elements in XY.
  • Your answer should be
  • an integer, like 6
  • a simplified proper fraction, like 3/5
  • a simplified improper fraction, like 7/4
  • a mixed number, like 1 3/4
  • an exact decimal, like 0.75
  • a multiple of pi, like 12 pi or 2/3 pi