Additional function for Loan Druid

[Nick]

Hi,

Given most of the loans I have had so far have been quoted by APR I 
wonder if there was a way of attaching the following code to a button 
within the Loan repayment druid. I'm just beginning to learn a little C, 
and can't figure out where to put it. Maybe someone else can:

int apr_to_simple_interest (float apr, int periods)
{
float simple_interest;
int periods;
float apr;

simple_interest = periods*((pow((1+apr),(1/periods)))-1);
}

Yours,
Nick