input: Write a function to find a pair of numbers in a list that sum up to a given value.
  assert find_pair([3, 4, 2, 8, 5, 6], 10) == (4, 6)
  assert find_pair([1, 2, 3, 9], 10) == (1, 9)
  assert find_pair([10, 20, 30, 40, 50], 70) == (20, 50)
output: def find_pair(numbers, target):
      seen = {}
      for number in numbers:
          complement = target - number
          if complement in seen:
              return (complement, number)
          seen[number] = True