This site gives a formula for a "constant ratio" as

MC=I/{1-[1/(1+I)ยช]}

Where:

MC is the mortgage constant

I is the interest per month (1/12 the annual rate)

a is the period in months.

It is not possible to solve this to give a formula for the interest rate. Iterative methods can be used to obtain an approximate value of I.

For your loan numbers, the annual percentage rate (12i) is approximately 7.19%.

MC=I/{1-[1/(1+I)ยช]}

Where:

MC is the mortgage constant

I is the interest per month (1/12 the annual rate)

a is the period in months.

It is not possible to solve this to give a formula for the interest rate. Iterative methods can be used to obtain an approximate value of I.

For your loan numbers, the annual percentage rate (12i) is approximately 7.19%.