gistfile1.rb

class FutureValue
  def initialze
  end

  def annual(rate, years, annual_contribution=0, initial_investment)
    pv = self.pv(rate, annual_contribution, years, initial_investment)
    pv * (1 + rate)**years
  end

  # where R is the periodic yield, A is the periodic payment,
  # N is the number of payments and i is the initial investment
  def pv(r, a, n, i)
    ((a / r) - a / (r * (1 + r)**n)) + i
  end
end

@fv = FutureValue.new
@fv.annual(0.03, 5, 1200, 10000) # => 17963.70