input: Write a python function to determine the minimum number of coins needed to make a given amount.
  assert min_coins(11) == 2
  assert min_coins(30) == 2
  assert min_coins(7) == 3
output: def min_coins(amount):
      coins = [25, 10, 5, 1]
      num_coins = 0
      for coin in coins:
          num_coins += amount // coin
          amount %= coin
      return num_coins