void writeGrade( const string & sName, const string & sGrade )
{
    .... open file in append and write name and grade ....
}

string calcGrade( double dGrade )
{
   string sGrade;
   .... do switch statement or whatever to get grade range ....
   return( sGrade );
}
void calcAvg( const string & sName, int iTest1, int iTest2, int iTest3, int iTest4, int iTest5 )
{
    double dGrade = ( iTest1 + iTest2 + iTest3 + iTest4 + iTest5 ) / 5;
    string sGrade = calcGrade( dGrade );
    writeGrade( sName, sGrade );
}
void readData( const string & sFile )
{
    ... open and read file, for each loop do ...
    {
        calcAvg( sName, iTest1, iTest2, iTest3, iTest4, iTest5 );
    }
}

int main( int argc, char **argv )
{
    readData( "grades.txt" );
    return( 0 );
}