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, 2 kg of each variety sells for the following prices:$6.50, $10.50, $11.25, and $12.50, 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 $6.50 immediately. If he ages the cheese, he must give up the $6.50 today to receive a higher amount in the future.

What is the IRR (expressed in percent per month) of the investment of giving up $65.00 today by choosing to store 20 kg of cheese that is currently 2 months old and instead selling 10 kg of this cheese when it has aged 9 months, 6 kg when it has aged 15 months, and the remaining 4 kg when it has aged 2 years?