Missing Number Puzzles Questions

19. What number comes next in this sequence?
  What is the next number?
Answer
Ans = 27.
Add, then deduct, the sum of the digits of the previous number alternately each time, ie 36 -3-6 = 27.
20. What number should replace the question mark?
  What is the next number?
Answer
6.
123456789101112131415


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