Write a main method that prompts the user for an integer between 1 & 20 (inclusive). if the user enters an invalid number, prompt for another one. when a valid number is entered, call activity with that number as a parameter.

int main() {

   int num;

   cout<<"Enter the number"<<endl;

   cin>>num;

   if(num<1 || num>20)

   {

       while(num>20 || num<1)//taking input if the number is not within range..

       {

           cout<<"Enter the number again"<<endl;

           cin>>num;

       }

       activity(num);//calling activity with the num..

   }

return 0;

}

Explanation:

Above written is the main function in which a prompting an integer between 1 and 20 if it is not within range then again taking input.Then after that calling activity with num as argument.