input: Write a function to find all unique pairs of numbers that sum up to a given target.
  assert find_combos(10)==[(0, 10), (2, 8), (4, 6), (6, 4), (8, 2), (10, 0)]
  assert find_combos(6)==[(0, 6), (2, 4), (4, 2), (6, 0)]
  assert find_combos(12)==[(0, 12), (2, 10), (4, 8), (6, 6), (8, 4), (10, 2), (12, 0)]
output: def find_combos(target):
      result = []
      for i in range(0, target+1, 2):
          result.append((i, target - i))
      return result