input: Write a python function to count pairs (x, y) such that a * x + b * y equals c.
  assert countPairs(2, 2, 4) == 3
  assert countPairs(1, 1, 2) == 3
  assert countPairs(3, 5, 15) == 2
output: def countPairs(a, b, c):
      count = 0
      for x in range(c // a + 1):
          if (c - a * x) % b == 0:
              count += 1
      return count