cannot change the value of base class variable from derived class - c++

Question

Hello I'm a bit confused of this problem I'm currently practicing to enhance my Inheritance task in c++. Would you mine to help me.
My goal is to update the acct_balance variable which is from base class.
#include <iostream>
class Account{
public:
Account(double initial_balance)
{
if(initial_balance >= 0)
acct_balance = initial_balance;
else
std::cout << "Invalid Balance!" << std::endl;
}
void credit(double x)
{
acct_balance += x;
}
void debit(double x)
{
if(x <= acct_balance){
acct_balance -= x;
flagWidraw = true;
}
else
std::cout << "Debit amount exceeded account balance." << std::endl;
}
double getBalance()
{
return acct_balance;
}
public:
double acct_balance;
bool flagWidraw = false;
};
class SavingsAccount:public Account{
public:
double interest_rate;
SavingsAccount(double balance, double interest):Account(balance)
{
interest_rate = interest;
}
double calculateInterest (){
return (acct_balance * (interest_rate/100));
}
};
class CheckingAccount:public Account{
public:
double charged_fee;
CheckingAccount(double balance, double fee):Account(balance)
{
charged_fee = fee;
}
void credit()
{
acct_balance -= charged_fee;
std::cout << "Checking Account Credit deduction fee "<< Account::acct_balance << std::endl;
}
void debit()
{
if(flagWidraw == true)
acct_balance -= charged_fee;
std::cout << "Checking Account Debit deduction fee "<< acct_balance << std::endl;
}
};
int main()
{
Account a(500.00);
SavingsAccount sa(a.getBalance(),12); //fixed interest rate 12
double interest = sa.calculateInterest();
std::cout << "Interest amount is added to your credit: " << interest << std::endl;
a.credit(interest);
CheckingAccount ca(a.getBalance(), 30);
ca.credit();
std::cout << "Balance is: " << a.getBalance() << std::endl;
return 0;
}