Answers to 9-hour clock questions

To find the standard name of a positive number, add the digits. If the sum is more than 9, repeat until you get a number small enough. (Remember that 9 = 0 (mod 9).) For negative numbers, do the same to get a negative number, then add 9.
Example: -37 = -(3+7) = -10 = -(1 + 0) = -1 =-1 + 9 = 8 (mod 19).

This process is known as casting out nines. For calculations, you can cast out nines at any point.

Answers to addition and subtraction questions.

  1. 3
  2. 3
  3. 4
  4. 1
  5. 8
  6. 5
  7. 8
  8. 1
  9. 6
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