> 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's quiz is to calculate the IRR for any given variable-length lis=
t 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...


I think one point, which isn't brought out here and not well in the
wikipedia article either, is that given all of the C_t, you still have
two unknowns: IRR (which we are attempting to solve for) and NPV.

In this case, you want NPV set to zero in order to solve for IRR.  Or
did I miss something?