1. in_file.open("input.dat");

2. out_file.close();

3. The following statements would not be needed: 
     ofstream out_file;                         
     out_file.open("output.dat");              
     out_file.close();

   Replace the following:  
     out_file << "first " << num1 << " second " << num2 
              << " third " << num3 << endl;

   with the following:
     cout << "first " << num1 << " second " << num2 
          << " third " << num3 << endl;

4. char file_name[16];
   cout << "Enter file name: ";
   cin >> file_name; 

5. 1020304011531

6. 65666746

   Note: The sequence above corresponds to the four characters "ABC."