For 6 months of the year he cuts his yard 15 days per month so he cuts his yard 6*15 = <<6*15=90>>90 times
For the remaining 6 months of the year, he cuts his yard three times a month so he cuts his yard 6*3 = <<6*3=18>>18 times
He cuts his yard 90 times for part of the year and 18 times for the rest of the year for a total of 90+18 = <<90+18=108>>108 times
There are 12 months in a year and he cuts his yard 108 times so he averages 108/12 = <<108/12=9>>9 times per month
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