Calculate payments for getting paid off in a certain time

loan_payment.rb

#!/usr/bin/env ruby
require 'date'

class Repayment
  MONTHLY_INTEREST = (<INTEREST (5.00)> / 12 / 100)

  def initialize(payment, principal)
    @payment, @principal = payment, principal
  end

  def self.summarize(principal, payments)
    old_repayment_count = 0

    payments.each do |payment|
      repayment = Repayment.new payment, principal
      maybe_summarize repayment, old_repayment_count
      old_repayment_count = repayment.number_of_payments
    end
  end

  def summarize
    puts
    puts '-' * 40
    puts 'Payment: $ %.2f' % payment
    puts 'Payments: %.2d' % number_of_payments
    puts 'Paid off: %s' % (Date.today >> number_of_payments).strftime("%b '%y")
  end

  def number_of_payments
    @number_of_payments ||= calculate_number_of_payments
  end

  private
  attr_reader :payment, :principal

  def self.maybe_summarize repayment, num_payments
    repayment.summarize unless repayment.number_of_payments == num_payments
  end

  def calculate_number_of_payments
    numerator = -1 * Math.log10(1 - MONTHLY_INTEREST * principal / payment)
    denominator = Math.log10(1 + MONTHLY_INTEREST)
    (numerator / denominator).ceil
  end
end

payments = (<MIN PAYMENT..<MAX PAYMENT>).step(0.01)
principal = <LOAN AMOUNT>
Repayment.summarize(principal, payments)