/*  Calculate what the interest rate per year would
**  have to be on a loan if the amount borrowed was
**  $10,000 for a period of 48 months and payments
**  were $250 per month. Because iper returns
**  interest/month in this case, it must be multiplied
**  by 12 to get interest per year. Also the payment
**  must be entered as a negative since it is a cash
**  outflow. Multiply by 100 to get a percentage.
*/
interest = 100*12*iper(pmt = -250, pv = 10000, nper = 48);