3.35 Use rand to produce x times y. Check answer. Repeat until correct.
3.36 Use random # generator to vary responses. Use switch to issue responses.
3.37 Count correct/incorrect; calculate %. Print string if <75%.
*/

#include <iostream>

using std::cout;
using std::cin;
using std::endl;

#include <cstdlib>

#include <ctime>

void good( void ); // response randomizers
void bad( void );

int main()
{
     int n,
     x,
     y,
     z = 0,
     correct = 0,
     incorrect = 0;
     srand( time (0) ); // random number generator
     cout << "How many questions? ";
     cin >> n;
     for ( int i = 1;      i <= n; )
     {
          x = ( 3 + rand() % 10 );
          y = ( 3 + rand() % 10 );
          cout << "How much is " << x << " x " << y << "? ";
          cin >> z;
          while ( z != x * y ) // repeats until correct
          {
               ++incorrect;
               bad();
               cout << x << " x " << y << " = ";
               cin >> z;
               i++;
          }
          if ( z == x * y )
          {
               ++correct;
               good();
               i++;
          }
     }
     cout << "Your score is " << ( correct * 100 ) / ( correct + incorrect ) << "%." << endl;
     if ( correct / 7.5 < incorrect / 2.5 ) // passing grade is 75%
     cout << "Please ask your instructor for extra help." << endl;
     return 0;
}

void good( void )
{
     switch ( rand() % 4 )
     {
     case 0: cout << "Very good!";
          break;
     case 1: cout << "Excellent!";
          break;
     case 2: cout << "Nice work!";
          break;
     case 3: cout << "Keep up the good work!";
          break;
     }
     cout << endl;
}

void bad( void )
{
     switch ( rand() % 4 )
     {
     case 0: cout << "No. Please try again.";
          break;
     case 1: cout << "Wrong. Try once more.";
          break;
     case 2: cout << "Don't give up!";
          break;
     case 3: cout << "No. Keep trying.";
          break;
     }
     cout << endl;
}