Real number system

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Across
  1. 1. all natural numbers including zero
  2. 3. the set of rational numbers together with the set of irrational numbers
  3. 5. positive and negative whole numbers
Down
  1. 2. all numbers used for counting objects
  2. 3. numbers that can be expressed as the ratio a/b, where "a" and "b" are integers and "b" does not equal zero
  3. 4. numbers that can't be expressed as the ratio a/b, where "a" and "b" are integers and "b" does not equal zero