What's the hardest (longest-running) financial calculation?

[ttw]

As rprosperi wrote, solving for the interest rate is the hardest as there is no closed-form formula. It's also possible to have inputs that do not allow for a solution. (One gets negative rates which are possible but rare, or one can get complex solutions which are unrealistic.) It's even worse computing rate of return (essentially an interest rate) with irregular payments.