The Laiterie de Coaticook in the Eastern Townships of Quebec produces several types of cheddar cheese. It markets this cheese in four varieties: aged $2$ months, $9$ months, $15$ months, and $2$ years. At the producer's store, $2kg$ of each variety sell for the following prices: $$8\mathrm{.}00,$$$10\mathrm{.}00,$$$11\mathrm{.}50,$ and $$12\mathrm{.}50,$ respectively. Consider the cheese maker's decision whether to continue to age a particular $2-kg$ block of cheese. At $2$ months, he can either sell the cheese immediately or let it age further. If he sells it now, he will receive $$8\mathrm{.}00$ immediately. If he ages the cheese, he must give up the $$8\mathrm{.}00$ today to receive a higher amount in the future.

What is the IRR $($expressed in percent per month$)$ of the investment of giving up $$80\mathrm{.}00$ today by choosing to store $20kg$ of cheese that is currently $2$ months old and instead selling $10kg$ of this cheese when it has aged $9$ months, $6kg$ when it has aged $15$ months, and the remaining $4kg$ when it has aged $2$ years?