input: 
  def count_pairs(numbers: List[int], x: int) -> int:
      """
      Count the number of pairs in the given list of numbers whose sum is equal to x.
      >>> count_pairs([1, 2, 3, 4, 5], 5)
      2
      >>> count_pairs([1, 2, 3, 4, 5], 10)
      0
      """
      count = 0
      i, j = 0, len(numbers) - 1
      while i < j:
          sum = numbers[i] + numbers[j]
          if sum == x:
              count += 1
              i += 1
          elif sum < x:
              i += 1
          else:
              j -= 1
      return count
  
output:     count = 0
      seen = set()
      for num in numbers:
          complement = x - num
          if complement in seen:
              count += 1
          seen.add(num)
      return count