Here's one way, that uses the method Fixnum#divmod to advantage:

COINS = [["H", 50], ["Q", 25], ["D", 10], ["N", 5], ["P", 1]]

def change(amount)
  raise ArgumentError, 'Only non-negative integers permitted' unless
    amount.is_a?(Fixnum) && amount >= 0
  return [] if amount.zero?
  amount_remaining = amount
  COINS.map do |label, value|
    nbr, amount_remaining = amount_remaining.divmod(value)
    [nbr, label]
  end
end

change(260) #=> [[5, "H"], [0, "Q"], [1, "D"], [0, "N"], [0, "P"]]
change(95)  #=> [[1, "H"], [1, "Q"], [2, "D"], [0, "N"], [0, "P"]]
change(100) #=> [[2, "H"], [0, "Q"], [0, "D"], [0, "N"], [0, "P"]]
change(80)  #=> [[1, "H"], [1, "Q"], [0, "D"], [1, "N"], [0, "P"]]
change(45)  #=> [[0, "H"], [1, "Q"], [2, "D"], [0, "N"], [0, "P"]]
change(16)  #=> [[0, "H"], [0, "Q"], [1, "D"], [1, "N"], [1, "P"]]
change(1)   #=> [[0, "H"], [0, "Q"], [0, "D"], [0, "N"], [1, "P"]]
change(0)   #=> []
change(-3)  #=> ArgumentError: Only non-negative integers permitted...
change(1.2) #=> ArgumentError: Only non-negative integers permitted...
change('c') #=> ArgumentError: Only non-negative integers permitted...

Here's one way, that uses the method Fixnum#divmod to advantage:

COINS = [["H", 50], ["Q", 25], ["D", 10], ["N", 5], ["P", 1]]

def change(amount)
  raise ArgumentError, 'Only non-negative integers permitted' unless
    amount.is_a?(Fixnum) && amount >= 0
  return [] if amount.zero?
  COINS.map do |label, value|
    nbr, amount = amount.divmod(value)
    [nbr, label]
  end
end

change(260) #=> [[5, "H"], [0, "Q"], [1, "D"], [0, "N"], [0, "P"]]
change(95)  #=> [[1, "H"], [1, "Q"], [2, "D"], [0, "N"], [0, "P"]]
change(100) #=> [[2, "H"], [0, "Q"], [0, "D"], [0, "N"], [0, "P"]]
change(80)  #=> [[1, "H"], [1, "Q"], [0, "D"], [1, "N"], [0, "P"]]
change(45)  #=> [[0, "H"], [1, "Q"], [2, "D"], [0, "N"], [0, "P"]]
change(16)  #=> [[0, "H"], [0, "Q"], [1, "D"], [1, "N"], [1, "P"]]
change(1)   #=> [[0, "H"], [0, "Q"], [0, "D"], [0, "N"], [1, "P"]]
change(0)   #=> []
change(-3)  #=> ArgumentError: Only non-negative integers permitted...
change(1.2) #=> ArgumentError: Only non-negative integers permitted...
change('c') #=> ArgumentError: Only non-negative integers permitted...