function compute_payment(balance, rate, num_payments, pay_per_year) {
  var payment;

  if (rate <= 0.0) {
    payment = balance * 1.0/(num_payments);
  } else {
    payment = balance * (rate/100.0/pay_per_year) /
      (1.0 - Math.pow(1.0 + rate/100.0/pay_per_year, -num_payments));
  }

  return Math.floor(payment * 100.0 + 0.5)/100.0;
}