stamps.rb

class Stamps
  attr_reader :amount

  def initialize(amount)
    if (amount =~ /^\d+$/).nil?
      raise ArgumentError, "The amount should be an integer"
    end

    if amount.to_i < 8
      raise ArgumentError, "The amount should be bigger than 7"
    end

    @amount = amount.to_i
  end

  def print_calculation
    fives, threes = calculate

    puts "#{fives} five cent stamp#{"s" if fives != 1}, #{
threes} three cent stamp#{"s" if threes != 1}"
  end

  def calculate
    remainder = amount % 5
    three_remainder = remainder % 3

    [amount/5 - three_remainder, (remainder/3) + three_remainder * 2]
  end
end

stamps_test.rb

require_relative 'stamps'
require 'test/unit'

class StampsTest < Test::Unit::TestCase
  def test_raise_empty
    assert_raise(ArgumentError) { Stamps.new(nil) }
  end

  def test_raise_lesser_than_8
    assert_raise(ArgumentError) { Stamps.new("4") }
  end

  def test_raise_non_integer
    assert_raise(ArgumentError) { Stamps.new("14.3") }
  end

  def test_raise_non_numerical
    assert_raise(ArgumentError) { Stamps.new("asd") }
  end

  def test_calculate_11
    stamps = Stamps.new(11.to_s)
    assert_equal([1, 2], stamps.calculate)
  end

  def test_calculate_100
    stamps = Stamps.new(100.to_s)
    assert_equal([20, 0], stamps.calculate)
  end

  def test_calculate_101
    stamps = Stamps.new(101.to_s)
    assert_equal([19, 2], stamps.calculate)
  end

  def test_calculate_10000002
    stamps = Stamps.new(10000002.to_s)
    assert_equal([1999998, 4], stamps.calculate)
  end
end

stampy.rb

#!/usr/bin/env ruby
require_relative 'stamps'

begin
  Stamps.new(ARGV[0]).print_calculation
rescue ArgumentError => e
  puts <<-EOF
  #{e.message}

  Usage:
    stampy.rb amount

  Options:
    amount - should be an integer lesser than 8

  Example:
    stampy.rb 15
  EOF
end