The errors i am getting are:

Error 1 error LNK2019: unresolved external symbol "double __cdecl getDollarAmt(void)" (? getDollarAmt@@YANXZ) referenced in function _main hid.obj Error 2 fatal error LNK1120: 1 unresolved externals

this is my program:

 #include<iostream>
#include<cmath>
#include<string>
using namespace std;

double getDollarAmt();
void displayCurrencies();
char getCurrencySelection (float amtExchanged);
bool isSelectionValid(char selection);
double calcExchangeAmt (float amtExchanged, char selection);
void displayResults(double newAmount, float amtExchanged, char selection, char yesNo);

const double  russianRubles = 31.168;
const double northKoreanWon = .385;
const double chineseYuan = 6.832;
const double canadianDollar = 1.1137;
const double cubanPeso = 1.0;
const double  ethiopianBirr = 9.09;
const double egyptianPound = 5.6275;
const double tunisianDinar = 1.3585;
const double thaiBaht = 34.4;

/****** 
I changed the variables to global variables so 
you don't have to worry about accidentally setting them
to 0 or assigning over a value that you need 
********/
float amtEchanged = 0.0;
char selection;
char yesNo;
double newAmount;

int main()
{
float amtExchanged = 0.0;
    selection = 'a';
    yesNo = 'y';
    newAmount = 0.0;

    getDollarAmt ();
    displayCurrencies();
    getCurrencySelection (amtExchanged);
    isSelectionValid(selection);/**** you only need to use the selection variable ****/
    calcExchangeAmt (amtExchanged, selection);
    displayResults(newAmount, amtExchanged, selection, yesNo);

    return 0;
}


double getDollarAmt (float amtExchanged)
// promt user for eachange amount and return it to main
{
    float amtExchanged0;//created temporary variable to set amtExchanged to

    cout<< "Please enter the total dollar amount to exchange:  ";
    cin>> amtExchanged0;


    amtExchanged = amtExchanged0;//setting amtExchanged to the right value
    return amtExchanged;
}


void displayCurrencies()
// display list of currencies
{
    cout<<"A    Russian Ruble"<<endl
        <<"B    North Korean Won"<<endl
        <<"C    Chinese Yuan"<<endl
        <<"D    Cuban Peso"<<endl
        <<"E    Ethiopian Birr"<<endl
        <<"F    Thai Baht"<<endl
        <<"G    Canadian Dollars"<<endl
        <<"H    Tunisian Dinar"<<endl
        <<"I    Egyptian Pound"<<endl;
}



char getCurrencySelection (float amtExchanged)
// make a selection and return to main
{
    char selection0;//again, created a temporary variable for selection

    cout<<"Please enter your selection:  ";
    cin>>selection0;


    selection = selection0;//setting the temporary variable to the actual variable you use

    /*****
    we are now going to see if isSelectionValid returns false.
    if it returns false, that means that their selection was not
    character A-H. if it is false we keep calling getCurrencySelection
    *****/
    if(isSelectionValid(selection)==false)
    {
        cout<<"Sorry, the selection you chose is invalid."<<endl;
        getCurrencySelection(amtExchanged);
    }


return selection;   
}


bool isSelectionValid(char selection) 
// this fuction is supposed to be called from getCurrencySelection, the selction
// must be sent to isSelectionValid to determine if its valid
// if selection is valid send it bac to getCurrencySelection
// if it is false then it is returned to getCurrencySelection and prompted to 
// make another selection until the selection is valid, then it is returned to main.
{
    /**** 
    i'm not sure if this is what you mean, 
    all i am doing is making sure 
    that their selection is A-H 
    *****/
    if(selection=='A' || selection=='B' || selection=='C' || selection=='D' || selection=='E' || selection=='F' || selection=='G' || selection=='H' || selection=='I')
        return true;
    else
        return false;   
}



double calcExchangeAmt (float amtExchanged,char selection)
// function calculates the amount of money to be exchanged
{

    switch (toupper(selection))
    {
    case 'A': newAmount =(russianRubles) * (amtExchanged);
        break;
    case 'B': newAmount = (northKoreanWon) * (amtExchanged);
        break;
    case 'C': newAmount = (chineseYuan) * (amtExchanged);
        break;
    case 'D': newAmount = (canadianDollar) * (amtExchanged);
        break;
    case 'E': newAmount = (cubanPeso) * (amtExchanged);
        break;
    case 'F': newAmount = (ethiopianBirr) * (amtExchanged);
        break;
    case 'G': newAmount = (egyptianPound) * (amtExchanged);
        break;
    case 'H': newAmount = (tunisianDinar) * (amtExchanged);
        break;
    case 'I': newAmount = (thaiBaht) * (amtExchanged);
        break;
    }
    return newAmount;
}


void displayResults(double newAmount, float amtExchanged, char selection, char yesNo)
// displays results and asked to repeat. IF they want to repeat it clears the screen and starts over.
{
    switch(toupper(selection))
    {
    case 'A': cout<<"$"<<amtExchanged<<" is "<<newAmount<<" Russian Rubles."<<endl<<endl;
        break;
    case 'B': cout<<"$"<<amtExchanged<<" is "<<newAmount<<" North Korean Won."<<endl<<endl;
        break;
    case 'C': cout<<"$"<<amtExchanged<<" is "<<newAmount<<" Chinese Yuan."<<endl<<endl;
        break;
    case 'D': cout<<"$"<<amtExchanged<<" is "<<newAmount<<" Cuban Pesos."<<endl<<endl;
        break;
    case 'E': cout<<"$"<<amtExchanged<<" is "<<newAmount<<" Ethiopian Birr."<<endl<<endl;
        break;
    case 'F': cout<<"$"<<amtExchanged<<" is "<<newAmount<<" Thai Baht."<<endl<<endl;
        break;  
    case 'G': cout<<"$"<<amtExchanged<<" is "<<newAmount<<" Canadian Dollars."<<endl<<endl;
        break;
    case 'H': cout<<"$"<<amtExchanged<<" is "<<newAmount<<" Tunisian Dinar."<<endl<<endl;
        break;
    case 'I': cout<<"$"<<amtExchanged<<" is "<<newAmount<<" Egyptian Pound."<<endl<<endl;
        break;
    }

    cout<<"Do you wish to continue?  (Y for Yes / N for No)";
    cin>>yesNo;

    if(yesNo=='y' || yesNo=='Y')
    {
        getDollarAmt(); 
    }
    else
    {
        system("cls");
    }

}