下から読まないと、つまんないよ

require 'singleton'

class Fact
  include Singleton
  def initialize
    @fact = Hash.new {|h, k| h[k] = k * h[k - 1]}
    @fact[1] = 1
  end

  def [](n)
    return 1 if n <= 1
    @fact[n]
  end
end

class Deck
  def initialize(seed, size=30)
    @fact = Fact.instance
    @seed = seed
    @size = size
  end

  def nCm(n, m)
    @fact[n].div(@fact[m] * @fact[n - m])
  end

  def zero
    card = @size - @seed
    nCm(card, 7).quo(nCm(@size, 7))
  end
end

d = Deck.new(5, 30)
puts "%.2f" % (d.zero * 100)

d = Deck.new(7, 30)
puts "%.2f" % (d.zero * 100)