Simple solution to Bloc's bank account exercise

bank_account.rb

class BankAccount
  attr_reader :balance

  def initialize(balance)
    @balance = balance
  end

  def display_balance
    @balance
  end

  def deposit(amount)
    @balance += amount if amount > 0
  end

  def withdraw(amount)
    @balance -= amount if amount <= @balance
  end
end

class CheckingAccount < BankAccount
  attr_reader :number_of_withdrawals
  MAX_FREE_WITHDRAWALS = 3
  WITHDRAWAL_FEE = 5

  def initialize(balance)
    super(balance)
    @number_of_withdrawals = 0
  end

  def get_free_withdrawal_limit
    MAX_FREE_WITHDRAWALS
  end

  def transfer(account, amount)
    if account.balance >= amount
      deposit(amount)
      account.withdraw(amount)
    end
  end

  def withdraw(amount)
    super(amount)
    @number_of_withdrawals += 1
    @balance -= WITHDRAWAL_FEE if number_of_withdrawals > 3
    if @balance < 0
      deposit(amount)
      deposit(WITHDRAWAL_FEE)
    end
  end
end