Here's my binary search solution.  It can return negative values
(extra credit) and returns nil for undefined behavior.

require "enumerator"
require "rubygems"
require "facets/numeric/round"

def npv(irr, cash_flows)
  cash_flows.enum_with_index.inject(0) do |sum, (c_t, t)|
    sum + c_t / (1+irr)**t
  end
end

def irr(cash_flows, precision=3D10 ** -4)

  # establish an upper bound, return nil if none
  max =3D 1.0
  max *=3D 2 until npv(max, cash_flows) < 0 or max.infinite?
  return nil if max.infinite?

  # initialize search variables
  last_irr, irr, radius =3D max, 0.0, max

  # binary search until precision is met
  until irr.approx?(last_irr, precision/10)
    last_irr =3D irr

    # improve approximation of irr
    if npv(irr, cash_flows) < 0
      irr -=3D radius
    else
      irr +=3D radius
    end

    # reduce the search space by half
    radius /=3D 2
  end

  irr.round_to(precision)
end

if __FILE__ =3D=3D $PROGRAM_NAME
  puts irr(ARGV.map { |e| Float(e) }) || "Undefined"
end

On Feb 8, 6:01 am, Ruby Quiz <ja... / grayproductions.net> wrote:
> The three rules of RubyQuiz:
>
> 1.  Please do not post any solutions or spoiler discussion for thisquizunt=
il
> 48 hours have passed from the time on this message.
>
> 2.  Support RubyQuizby submitting ideas as often as you can:
>
> http://www.rubyquiz.com/
>
> 3.  Enjoy!
>
> Suggestion:  A [QUIZ] in the subject of emails about the problem helps eve=
ryone
> on Ruby Talk follow the discussion.  Please reply to the originalquizmessa=
ge,
> if you can.
>
> -=3D-=3D-=3D-=3D-=3D-=3D-=3D-=3D-=3D-=3D-=3D-=3D-=3D-=3D-=3D-=3D-=3D-=3D-=
=3D-=3D-=3D-=3D-=3D-=3D-=3D-=3D-=3D-=3D-=3D-=3D-=3D-=3D-=3D-=3D-=3D-=3D-=3D-=
 =3D-=3D-=3D
>
> by Harrison Reiser
>
> Internal Rate of Return (IRR -http://en.wikipedia.org/wiki/Internal_rate_o=
f_return) is a common financial
> metric, used by investment firms to predict the profitability of a company=
 or
> project. Finding the IRR of a company amounts to solving for it in the equ=
ation
> for Net Present Value (NPV -http://en.wikipedia.org/wiki/Net_present_value=
),
> another valuable decision-making metric:
>
>                N      C_t
>         NPV =3D  =D3  ------------
>               t=3D0 (1 + IRR)**t
>
> This week'squizis to calculate the IRR for any given variable-length list =
of
> numbers, which represent yearly cash flows, the C_t's in the formula above=
: C_0,
> C_1, etc. (C_0 is typically a negative value, corresponding to the initial=

> investment into the project.) From the example in the Wikipedia article
> (http://en.wikipedia.org/wiki/Internal_rate_of_return), for instance, you =
should
> be able to produce a rate of 17.09% (to four decimal places, let's say) fr=
om
> this or a similar command:
>
>         irr([-100,+30,+35,+40,+45])
>         =3D> 0.1709...
>
> Keep in mind that an IRR greater than 100% is possible. Extra credit if yo=
u can
> also correctly handle input that produces negative rates, disregarding the=
 fact
> that they make no sense.