Math problem in c++ program ugh. :)

[moriarty]

I have a function to determine the monthly payment on user input loan amount, interest rate, and length in months.

double calcPayment(double Principle, double Rate, double Term) {

  
  double MonthlyRate = Rate/12.0/100; 
  double t = pow((1 + MonthlyRate),Term); 
  double Payment = (Principle * MonthlyRate * t) / (t-1); //
  return(Payment); 

Now I need to add a function to determine interest paid for life of loan.
Which is where I'm just not hitting my stride.
I assumed incorrectly that it would be length of loan * interest rate * monthly payment.

This is not for my homework or anything like that, I give you my word (how many people you hear say that?) What it's for is my wife pseudo-code program that I want to write in c++ and while her teacher says the math doesn't matter, she just wants to see proper use of functions, I do care and want to know.

Should I, perhaps, run the quoted code in a loop while counting down until the loan is paid off and keeping track of the amount of interest paid per month as I go? I know this might be off topic and apologies if it is too far 'off'.

[jsmith]

The total amount of interest paid over the life of the loan is equal to the monthly payment times the number of
payments minus the loan amount.