Don't understand why output is incorrect - c++

Question

I'm currently doing a Zybooks lesson for my C++ class and we're going over while loops. In this question, it wants me to calculate how many years it takes for a bank account to double it's initial balance. There is also an annual contribution added. My code is as follows:
#include <iostream>
using namespace std;
int main()
{
const double RATE = 5;
const double INITIAL_BALANCE = 10000;
const double TARGET = 2 * INITIAL_BALANCE;
cout << "Annual contribution: " << endl;
double contribution;
cin >> contribution;
double balance = INITIAL_BALANCE;
int year = 0;
while (balance < TARGET)
{
year++;
double interest = balance * RATE / 100;
balance = balance + interest + contribution;
}
cout << "Year: " << year << endl;
cout << "Balance: " << balance << endl;
return 0;
}
I used this as an answer but was met with this unexpected result:
Output differs. See highlights below.
Input
100
Your output
Annual contribution:
Year: 13
Balance: 20627.8
Expected output
Annual contribution:
Year: 13
Balance: 20527.8

Answer 1

I see the expected output and your output differs by a 100, i.e. your contribution. Maybe the evaluation system doesn't add annual contribution once your target is reached. The below code gets your required output, but I think your code should have been the correct answer.
#include <iostream>
using namespace std;
int main()
{
const double RATE = 5;
const double INITIAL_BALANCE = 10000;
const double TARGET = 2 * INITIAL_BALANCE;
cout << "Annual contribution: " << endl;
double contribution;
cin >> contribution;
double balance = INITIAL_BALANCE;
int year = 0;
while (balance < TARGET)
{
year++;
double interest = balance * RATE / 100;
balance = balance + interest;
if (balance < TARGET) {
balance += contribution;
}
}
cout << "Year: " << year << endl;
cout << "Balance: " << balance << endl;
return 0;
}

Answer 2

The problem is that you're making a contribution after the 13th year even though the target has been reached.
I would restructure to something like this, in order to only check the condition once:
while (true)
{
year++;
double interest = balance * RATE / 100;
balance += interest;
if (balance >= TARGET)
{
break;
}
balance += contribution;
}

Answer 3

This is because the last time the loop is being executed (at year=13), balance is less than TARGET but, after adding a contribution and interest into it, it jumps out of the loop with contribution and interest added to it.
So, the solution can be to use an if statement within the while loop to check if it exceeds the TARGET; if it does, then don't add contribution into it.
Replace your while loop with the following:
while (balance < TARGET)
{
year++;
double interest = balance * RATE / 100;
balance = balance + interest;
if (balance < TARGET) {
balance += contribution;
}
}