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    Here is the program that determines the number of questions that the student answered correctly.

    #include <iostream>  //to use input output functions

    #include <fstream>  // to handle files

    #define size 20  //size of questions is fixed to 20

    using namespace std;   //to identify objects cin cout

    int correctAnswers(char answers[], char correct[]);  //to count correct answers given by student

       int main(){    //start of main function

           char correct[size];         // array stores correct answers

           char answers[size];       //stores answers of students

           int result;                     //stores the correct answers result of student

           double percentage;   //stores the percentage of correct answers

           int counter;        //counts number of correct answers given by student  

           ifstream reader("CorrectAnswers.txt");  //reads the contents of CorrectAnswers.txt file

           if(!reader){  //if file cannot be read/opened

               cout<<"Cannot open the file"<<endl; //displays error message

               return 0;        }          

           counter = 0;    //sets counter to 0 initially

           while( (reader>>correct[counter++]) && counter < size  );    //reads each answer from CorrectAnswers.txt  and stores it in char array correct[] at each iteration of while loop

           reader.close();  //closes the file

           reader.open("StudentAnswers.txt");    //opens StudentAnswers.txt file  

           if(!reader){   //if file could not be opened

               cout<<"Cannot open file"<<endl;   //displays this error message

               return 0;          }          

           counter = 0;    //sets counter to 0

           while( (reader>>answers[counter++]) && counter < size );   //reads each answer from StudentAnswers.txt  and stores it in char array answers[] at each iteration of while loop    

           result = correctAnswers(answers,correct);  //calls correctAnswers method to compute the correct answers given by student and stores the result of correct answers in result variable

           percentage = 100.0*result/size;   //computes percentage of correct answers of student    

           cout<<"The total number of questions answered correctly: "<<result<<endl;     //displays total no of qs answered correctly

           cout<<"The percentage of questions answered correctly: "<<percentage<<" %"<<endl;  //displays computed percentage

           if( percentage >= 75 ){  //if percentage of correctly answered questions is 75% or greater

               cout<<"Student has passed the exam!"<<endl;   } //displays that the student passed the exam

          else { //if percentage of correctly answered questions is below 75%

               cout<<"Student has failed the exam!"<<endl;         }  //displays that the student failed the exam

           return 0;     }  

       int correctAnswers(char answers[], char correct[]){   //method to compute the number of correct answers  

           int countCorrect = 0;   //stores the count of correct answers

           for(int i=0; i<size; i++){  //iterates 20 times to match answers from both the files of 20 questions

               if( answers[i] == correct[i] ){  // if the student answer in StudentAnswers.txt file matches with the correct answer given in CorrectAnswers.txt file

                   countCorrect++;              }         }     //adds 1 to the countCorrect every time the student answer matches with the correct answer

           return countCorrect;     }  //returns the number of questions that the student answered correctly

    Explanation:

    The program is well explained in the comments mentioned with each line of code.

    Here are the two files that can be used to check the working of the above program. You can create your own text files too.

         CorrectAnswers.txt :

    A

    A

    B

    D

    C

    C

    A

    C

    D

    D

    A

    B

    A

    B

    B

    C

    A

    C

    C

    B

    StudentAnswers.txt

    A

    C

    B

    D

    C

    C

    B

    C

    D

    C

    D

    B

    A

    B

    B

    C

    A

    C

    C

    B      

    Also if the number of wrong answers are to be displayed then add this method to the above code:

    void wrongAnswers(char answers[], char correct[]){  //method takes two arrays one with correct answers and the other with student answers

           for(int i=0; i<size; i++){ //iterates 20 times

               if( answers[i] != correct[i] ){ //if the answer by student does not match with correct answer

                   cout<<"Question Number: "<<i+1<<endl; //the question that is answered incorrectly

                   cout<<"Correct answer is: "<<correct[i]<<endl;  //displays the correct answer

                   cout<<"Student answer is: "<<answers[i]<<endl;    }    }    } //displays the answer that student gave

    Now call this method in main() as:

    if(result < size){  //if result is less than size i.e. 20

               cout<<"The questions answered incorrectly"<<endl;  //number of questions that are answered incorrectly

               wrongAnswers(answers,correct);  //calls method to compute the wrongly answered questions

           }else{  //if result is not less than size

               cout<<"No incorrect answers."<<endl;       }    //no questions incorrectly answered      

           cout<<"Number of missed questions: "<<size-result<<endl; //displays the number of missed questions

    The screenshot of the output is attached.

    [tex]One of your professors has asked you to write a program to grade her midterm exams, which consist of[/tex]
    [tex]One of your professors has asked you to write a program to grade her midterm exams, which consist of[/tex]

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