You can take the Rathore out of Ranjhore but you can never take out the Ranjhore out of Rathore

and similarly - "You can take the engineer out of engineering but you can not take the engineering out of an engineer" :)

Just spent 2 hours of my precious time deriving the formula for IRR (Internal Rate of Return) . Yes I could have found it on the net , but that wouldn't be fun would it now .

so here we go
P = Loan amount
I = Yearly installment
N = Number of years
R = Annual rate of interest
B = (1 + I/100) => for I = 10% , B= 1.1

I = P / ( B^-1 + B^-2 + B^-3 ... B^-N)  => Eq-1

Hence for a given loan , years and rate of interest you could find out the yearly installment :)

Unfortunately I don't remember how to solve polynomial equations but you can also re-arrange the above equation to find out rate of interest if monthly installment is known.

( B^-1 + B^-2 + B^-3 ... B^-N) = P / I  => Eq-2

The above equation can be solved for "B" in excel using

"Data -> Data Tools -> What If Analysis -> Goal Seek "

oh yes btw excel gives you an easier formula for the above IRR() ;)

:D . You think I am crazy , believe me, I am ;)

Signing off , need to get some work done