class Account
  attr_accessor :balance
  def initialize(balance)
    @balance = balance
  end
end
class Transaction
  def initialize(account_a, account_b)
    @account_a = account_a
    @account_b = account_b
  end
private
  def debit(account, amount)
    account.balance -= amount
  end
  def credit(account, amount)
    account.balance += amount
  end
public
#...
  def transfer(amount)
    debit(@account_a, amount)
    credit(@account_b, amount)
  end
#...
end