Error" new_account was not declared in this scope

[Bopaki]

the following program is exactly as it is in the book by Walter Savitch but when I compile it I get the error in the title.

  //Program to demonstrate the class BankAccount.
#include<iostream>
using namespace std;

//Class for a bank account:
class BankAccount
{
public:
	void set(int dollars, int cents, double rate);
	//Postcondition: The account balance has been set to $dollars.cents;
	//The interest rate has been set to rate percent.
	
	void set(int dollars, double rate);
	//Postcondition: The account balance has been set to $dollars.00.
	//The interest rate has been set to rate percent.
	
	void update();
	//Postcondition: One year of simple interest has been
	//added to the account balance.
	
	double get_balance();
	//Returns the current account balance.
	
	double get_rate();
	//Returns the current account interest rate as a percentage.
	
	void output(ostream& outs);
	//Precondition: If outs is a file output stream, then
	//outs has already been connected to a file.
	//Postcondition: Account balance and interest rate have
	//been written to the stream outs.
	
	BankAccount new_account(BankAccount old_account);
	//Precondition: old_account has previously been given values
	//(that is, its member variables have been given values).
	//REturns the value for a new account that has a balance of zero
	//and the same interest rate as the old_account.
	
private:
	double balance;
	double interest_rate;
	
	double fraction(double percent);
	//Converts a percentage to a fraction. For example, fraction(50.3)
	//returns 0.503.
};

int main()
{
	BankAccount account1, account2;
	cout << "Start of Test:\n";
	account1.set(123, 99, 3.0);
	cout << "account1 initial statement:\n";
	account1.output(cout);
	account1.set(100, 5.0);
	cout << "account1 with new setup:\n";
	account1.output(cout);
	
	account1.update();
	cout << "account1 after update:\n";
	account1.output(cout);
	
	BankAccount account3, account4;
	account3.set(999, 99, 5.5);
	account4 = new_account(account3);
	account4.output(cout);
	
	return 0;
}

void BankAccount::set(int dollars, int cents, double rate)
{
	if ((dollars < 0) || (cents < 0) || (rate < 0))
	{
		cout << "Illegal values for money of interest rate.\n";
		exit(1);
	}
	
	balance = dollars + 0.01*cents;
	interest_rate = rate;
}

void BankAccount::set(int dollars, double rate)
{
	if ((dollars < 0) || (rate < 0))
	{
		cout << "Illegal values for money or interest rate.\n";
		exit(1);
	}
	
	balance = dollars;
	interest_rate = rate;
}

void BankAccount::update()
{
	balance = balance + fraction(interest_rate) * balance;
}

double BankAccount::fraction(double percent_value)
{
	return (percent_value/100.0);
}

double BankAccount::get_balance() 
{
	return balance;
}

double BankAccount::get_rate()
{
	return interest_rate;
}

//Uses iostream
void BankAccount::output(ostream& outs)
{
	outs.setf(ios::fixed);
	outs.setf(ios::showpoint);
	outs.precision(2);
	outs << "Account balance $" << balance << endl;
	outs << "Interest rate " << interest_rate << "%" << endl;
}

BankAccount new_account(BankAccount old_account)
{
	BankAccount temp;
	temp.set(0, old_account.get_rate() );
	return temp;
}

According to the author the program would produce the following output:
Account balance $0.00;
Interest rate 5.50%

I just can find where the problem is.

[Thomas1965]

new_account is declared as a member of the class BankAccount(Line 33),
but defined as a free function(Line 125).

[Bopaki]

On the summary of Some properties of Classes the last one that I am quoting as it is written in the textbook.

A function may return an object; that is, a class may be the type for the value returned by a function. (See Self-Test Exercise 21.)

Now how do I fix an error that is an example by the author?

[Thomas1965]

One way to fix it is to move lines 33-37 outside the class.

Another option is to change line 65 to account4 = account4.new_account(account3);
and change line 125 to BankAccount BankAccount::new_account(BankAccount old_account);

[Bopaki]

Thanks Thomas1965 the second option solved the problem.
Some of the problems we get here can be very difficult especially if it is wrong in the textbook.