Just so I don't forget and have to re-figure this another time in my life. This isn't optimized and does nothing to handle rounding errors.

amortization_table.rb

class AmortizationTable
  Transaction = Struct.new(:payment, :interest, :principle, :balance)

  attr_reader :amount, :rate, :n

  def initialize(opts={})
    @amount = opts.fetch(:amount, 25_000)
    @rate = opts.fetch(:rate, (0.06 / 36))
    @n = opts.fetch(:n, 36)
  end

  def table
    @table ||= n.times.inject({}) do |hash, i|
      last_payment = hash[i - 1]
      last_balance = last_payment ? last_payment.balance : amount
      new_interest = last_balance * rate
      new_principle = payment - new_interest
      new_balance = last_balance - new_principle
      new_payment = last_balance < payment ? last_balance : payment
      hash[i] = Transaction.new(new_payment, new_interest, new_principle, new_balance)
      hash
    end
  end
  
  def payment
    amount * ((((1 + rate)**n) * rate) / (((1 + rate) ** n) - 1))
  end
  
  def interest(month)
    index = month - 1
    table[index] ? table[index].interest : 0
  end
  
  def principle(month)
    index = month - 1
    table[index] ? table[index].principle : 0
  end
  
  def balance(month)
    index = month - 1
    table[index] ? table[index].balance : 0
  end
  
  def inspect
    "AmortizationTable amount: #{amount}, rate: #{rate}, n: #{n}"
  end
end