C++ Homework giving LNK2019: unresolved external symbol error

Hey guys I'm fairly new to C++ and wondering if you guys can help me with my homework?It's basically a money converter for pounds and euros but when I compile I get this:

1> HwkJohn.cpp 1>HwkJohn.obj : error LNK2019: unresolved external symbol "void __cdecl convertPriceIntoEuros(void)" (?convertPriceIntoEuros@@YAXXZ) referenced in function "void __cdecl processAPrice(void)" (?processAPrice@@YAXXZ) 1>HwkJohn.obj : error LNK2019: unresolved external symbol "void __cdecl showPriceInEuros(void)" (?showPriceInEuros@@YAXXZ) referenced in function "void __cdecl processAPrice(void)" (?processAPrice@@YAXXZ) 1>HwkJohn.obj : error LNK2019: unresolved external symbol "void __cdecl calculateSum(void)" (?calculateSum@@YAXXZ) referenced in function "void __cdecl processAPrice(void)" (?processAPrice@@YAXXZ) 1>HwkJohn.obj : error LNK2019: unresolved external symbol "void __cdecl displayFinalData(void)" (?displayFinalData@@YAXXZ) referenced in function "void __cdecl processAPrice(void)" (?processAPrice@@YAXXZ) 1>c:\users\john\documents\visual studio 2012\Projects\HwkJohn\Debug\HwkJohn.exe : fatal error LNK1120: 4 unresolved externals

spent a few hours googling this definition but I don't understand it if anyone could help? Were also not allowed to use any global variables in the program.

#include <iostream>         //for cin >> and cout <<
#include <cassert>          //for assert
#include <iomanip>  

using namespace std;

void processAPrice();
double getPriceInPounds();
void convertPriceIntoEuros();
void showPriceInEuros();
void calculateSum();
void displayFinalData();

int main()                  
{
    char answer;
    float numberOfPrices = 0.0;


    while (&(answer = 'Y') || (answer = 'y'))
    {
        processAPrice();
        numberOfPrices += 1;
        cout << "Continue (Y/N)";
        cin >> answer;
    }

        if (numberOfPrices > 0)
    {
    //  displayFinalData;

    }


return 0;

}

void processAPrice()
{
    getPriceInPounds();
    convertPriceIntoEuros();
    showPriceInEuros();
    calculateSum();
    displayFinalData();
}

double getPriceInPounds()
{
    double priceInPounds;

    cout << "Enter a price (in pounds): £";
    cin >> priceInPounds;

    return priceInPounds;
}

double convertPriceIntoEuros(double priceInPounds)
{
double priceInEuros = 0.0;
const double conversionRate = 0.82;

priceInEuros = priceInPounds / conversionRate;
return priceInEuros;
}

double showPriceInEuros(double priceInPounds, double priceInEuros)
{
    cout << "The euro value of £:", priceInPounds, " is €", priceInEuros;
    return priceInEuros;
}

double calculateSum(double priceInEuros)
{
    double sumInEuros = 0.;
    sumInEuros = sumInEuros + priceInEuros;
    return sumInEuros;
}

double displayFinalData(double numberOfPrice, double sumInEuros)
{
    cout << "The total sum is:", sumInEuros, "€";
    cout << "The average is: ", sumInEuros/numberOfPrice;

    return 0;
}