# Instalment APR Calculation Formula

I have a problem that I cannot solve regarding interest payments, and payment holidays. The scenario is this

If I purchase a product which is paid for over a 12 month period, I need to calculate the monthly interest payments. However, the complexity here, is that I have the option to take one month payment holiday (is optional, but can be any month over the 12 month period).

So far, I have

annual interest (a) = (e.g for 5%, a=5)

monthly interest (m) = ((1 + a/100)^(1/12) - 1) x 100

balance = balance*(1+m) - instalment


How do I calculate the instalment, especially when including payment holiday?

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