I'm trying to build a calculator to determine different Factors for specific Investments. I have a Formula to calculate the Grand Total, but I can't see any way to get either the Duration or the interest rate from this, as Long as there is both an Initial Investment and a monthly Payment. Unfortunatly I am no native Speaker, so I'm probably not searching for the correct phrases.
K: Grand Total
p: partial Interest(monthly)
d: Duration(in Months)
iv: Initial Investment
mv: monthly Investment
K(d) = (mv * (1 + p) * ((1 + p)^d - 1) / p) + iv * (1 + p)^d
Do you know a formula or other method to calculate either the Duration or the interest rate, assuming all other Factors are known?