Update iteration result - c++

Question

EDIT: Global variables are restricted as part of the assignment (Forgot to mention this).
The program is meant to calculate a projected population every year for a specified number of years. Here is the equation:
N = P(1 + B)(1 - D)
Where N is the new population size, P is the previous population size, B is the birth rate, and D is the death rate. B and D are decimals.
So my problem is that I can only get the first iteration's result, and I don't know how to update the result so that it is multiplied by the (1 + B) and (1 - D). My guess is that it has to do with one of the combined assignment operators, but I haven't had success.
#include <iostream>
#include <iomanip>
#include <cmath>
using namespace std;
int get_pop();
int get_numYears();
double get_annBirthRate();
double get_annDeathRate();
double pop_new(int& A, double& B, double& C, double& D);
int main()
{
int P,
num,
Y;
double B,
D,
N;
P = get_pop();
B = get_annBirthRate();
D = get_annDeathRate();
Y = get_numYears();
for (num = 1; num != Y; num++)
{
N = pop_new(P, B, D, N);
cout << "\n\nThe projected population for the end of year " << num << " is " << N << endl;
cin.ignore();
}
return 0;
}
int get_pop()
{
int pop;
cout << "Enter the starting population:" << endl;
cin >> pop;
while (pop < 2)
{
cout << "\n\nError - starting population cannot be less than 2:" << endl;
cin >> pop;
}
return pop;
}
double get_annBirthRate()
{
double annBirthRate;
cout << "\n\nEnter the annual birth rate \n(a positive percentage in decimal form):" << endl;
cin >> annBirthRate;
while (annBirthRate < 0)
{
cout << "\n\nError - annual birth rate cannot be negative:" << endl;
cin >> annBirthRate;
}
return annBirthRate;
}
double get_annDeathRate()
{
double annDeathRate;
cout << "\n\nEnter the annual death rate \n(a positive percentage in decimal form):" << endl;
cin >> annDeathRate;
while (annDeathRate < 0)
{
cout << "\n\nError - death rate cannot be negative:" << endl;
cin >> annDeathRate;
}
return annDeathRate;
}
int get_numYears()
{
int numYears;
cout << "\n\nEnter the number of years to display:" << endl;
cin >> numYears;
while (numYears < 0)
{
cout << "\n\nError - number of years cannot be less than 1:" << endl;
cin >> numYears;
}
return numYears;
}
double pop_new(int& P, double& B, double& D, double& N)
{
N = P * (1 + B) * (1 - D);
return N;
}

Answer 1

The values P, B, and D do not change.
Make it:
double pop_new(int P, double B, double D)
{
double N = P * (1 + B) * (1 - D);
return N;
}
and:
P = pop_new(P, B, D);
cout << "\n\nThe projected population for the end of year "
<< num << " is " << P << endl;
Note: You do not need to pass references (and if, you should have passed const double&)