The standard formula for installment amount A, for n periods with interest per period i, and present value (principal) P is
substitute to get
I do not believe there is an explicit formula to solve for i, so we can solve it by trial and error or by iteration.
Rearrange to give
and we look for i that makes f(i)=0.
Make a table:
Interpolating, we try
So give a final try of
The interest per period is 1.1045%
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