Answer:
1.075%
Explanation:
You want the monthly periodic rate corresponding to an APR of 12.9%.
Monthly rate
There are 12 months in a year, so the monthly rate is 1/12 of the annual rate:
12.9%/12 = 1.075%
The monthly periodic rate is 1.075%.
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Additional comment
The APR does not take compounding into account, so the effective annual rate may be higher.