Number system

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Across
  1. 3. numbers are denoted as
  2. 5. was the first person to discover the numbers which were not rational around 400BC
  3. 7. numbers are denoted as
  4. 8. whole number is a natural number
  5. 9. are denoted as
Down
  1. 1. many rational numbers are there between any two given rational numbers
  2. 2. number is neither a negative or positive
  3. 4. which start with 0 are called
  4. 6. which are used for counting