class LibraryCard {

private:

  int maxBooks;  // maximum number of books this user can borrow
  int numBooks;   // number of books currently borrowed
  
public:

  // constructs a LibraryCard object (for a patron).  The patron is allowed up to 20 books and currently has none borrowed.
  LibraryCard();

  // constructs a LibraryCard object (for a patron).  The patron is allowed to borrow up to the given number of books, and
  // currently has none signed out.
  // Throws invalid_argument exception if argument is less than or equal to 0.
  LibraryCard(int maxBooks);

  // numBooks books are signed out on the given library card.
  // if numBooks will cause the patron to have more than the maximum number of books, this method has no effect.
  // Throws invalid_argument exception if argument is less than 0.
  void borrowBooks(int numBooks);

  // numBooks books are returned on the given library card.
  // if numBooks is larger than the number of books this patron has, this method has no effect.
  // Throws invalid_argument exception if argument is less than 0.
  void returnBooks(int numBooks);

  // prints the number of books that this LibraryCard has currently signed out.
  // (Does not change the state of the LibraryCard.)
  void booksOut() const;

  // returns the percent level of the LibraryCard usage, i.e. 100 = max number of books signed out; 
  // 0 = no books signed out, etc.
  // (Does not change the state of the LibraryCard.)
  double currentUsage() const;

};