Solution to the mortgage calculator challenge

calculator.rb

class Mortgage
  
  attr_reader :principal, :down_payment_percentage, :apr, :duration_in_years

  def initialize(principal, down_payment_percentage, apr, years)
    @principal = principal
    @down_payment_percentage = down_payment_percentage
    @apr = apr
    @duration_in_years = years
  end

  def number_of_payments
    duration_in_years * 12
  end
  
  def down_payment
    down_payment_percentage * principal
  end
 
  def monthly_rate_decimal
    (apr/12) / 100
  end

  def monthly_payment
    # (monthly_rate_decimal * (principal - down_payment) ) / (1 - ((1 + monthly_rate_decimal) ** (-1 * number_of_payments)) )
    amount_owed / payment_coefficient #Both of these calculations yield the monthly payment
  end

  def amount_owed
    ( (1 + monthly_rate_decimal) ** number_of_payments ) * (principal - down_payment)
  end

  def payment_coefficient
    counter = 1
    sum = 1
    (number_of_payments.to_i - 1).times do
      sum += (1 + monthly_rate_decimal) ** counter
      counter += 1
    end
    sum 
  end

  def principal_payment
    (principal - down_payment) / number_of_payments
  end

  def total_interest_paid
    (monthly_payment * number_of_payments) - principal
  end

end

# require_relative 'mortgage'

DOWN_PAYMENT_PERCENTAGES = [0.05, 0.10, 0.15, 0.20]

print "What is the home value: "
home_value = gets.chomp.to_f

print "What is the APR: "
apr = gets.chomp.to_f

print "What is the duration (in years): "
duration_in_years = gets.chomp.to_f

puts
puts sprintf("%-15s %-10s %-10s %-20s %-20s",
  "Down Payment", "Duration", "APR", "Monthly Payment", "Total Interest Paid")

DOWN_PAYMENT_PERCENTAGES.each do |down_payment_percentage|
  mortgage = Mortgage.new(home_value, down_payment_percentage, apr, duration_in_years)

  puts sprintf("%-15.2f %-10d %-10.3f %-20.2f %-20.2f",
    mortgage.down_payment, duration_in_years, apr,
    mortgage.monthly_payment, mortgage.total_interest_paid)
end