error LNK2019

[jdmay]

I have the following code:
# include <iostream>
# include <iomanip>
using namespace std;
using std::cin;
using std::cout;
using std::endl;

//function prototypes
double calcAverage (double, double, double);
void getTestScore(double&, double&, double&);
void displayAverage (double);



//main
int main ()
{
double score_1 = 0.0;
double score_2 = 0.0;
double score_3 = 0.0;
double average = 0.0;

//get score inputs from getTestScores
getTestScore(score_1, score_2, score_3);

//calculate average
average = calcAverage (score_1, score_2, score_3);

//display average
displayAverage (average);

return 0;
}

//**********function definitions**********

//getTest Scores
void getTestScore(double& score_1, double& score_2, double& score_3)
{
cout << "Enter score 1:" << endl;
cin >> score_1;
cout << "Enter score 2:" << endl;
cin >> score_2;
cout << "Enter score 3:" << endl;
cin >> score_3;
}//end getTestScore

//calcAverage
double calcAverage (double& score_1, double& score_2, double& score_3)
{
double average = 0.0;
average = (score_1 + score_2 + score_3) / 3.0;
return average;
}//end calcAverage

//displayAverage
void displayAverage (double& avgerage)
{
cout << "The average is " << avgerage <<"." <<endl;
}//end displayAverage


I am getting the errors:
error LNK2019: unresolved external symbol "void __cdecl displayAverage(double)" (?displayAverage@@YAXN@Z) referenced in function _main

and

error LNK2019: unresolved external symbol "double __cdecl calcAverage(double,double,double)" (?calcAverage@@YANNNN@Z) referenced in function _main

I am relatively new to C++ and do not know how to fix this error.

[Zhuge]

The function prototypes probably don't match the definitions.

[jdmay]

I've tried modifying the prototypes and function calls but they seem to match.

[writetonsharma]

you are missing & in the .function declarations.

Also, built in types are better passed as value compared to references if you are not to be changed.