Solução para o desafio opcional, com o intuito de praticar a orientação a objetos:

package Bank;

public class BankAccount {

    private double totalBalance;

    public void deposit(double amount) {
        System.out.println("Depositing: " + amount);
        totalBalance += amount;
    }

    public void withdraw(double amount) {
        System.out.println("Withdrawing: " + amount);
        if (amount <= totalBalance) {
            totalBalance -= amount;
        } else {
            System.out.println("Insufficient funds");
        }
    }

    public void showBalance() {
        System.out.println("Total balance: " + totalBalance);
    }

}

package Bank;

public class CheckingAccount extends BankAccount {


    public void chargeMonthlyFee() {
        System.out.println("Charging monthly fee");
        double monthlyFee = 10.0;
        withdraw(monthlyFee);
    }
}

import Bank.CheckingAccount;
import java.util.Scanner;

public class Main extends PrimeNumbers {
    public static void main(String[] args) {

        CheckingAccount newAccount = new CheckingAccount();
        newAccount.deposit(200.0);
        newAccount.withdraw(50.0);
        newAccount.showBalance();
        newAccount.chargeMonthlyFee();
        newAccount.showBalance();

    }
}