An ISBN has thirteen digits. The first twelve identify the country in which the book was printed, the publisher, and the individual book. The thirteenth digit is the "check digit." It is chosen in such a way that (d1 + 3d2 + d3 + 3d4 + ... + d11 + 3d12 + d13) mod 10 = 0 where d1, d2, d3..., d13 are digits of the ISBN. mod stands for modulo division (same as % in Java for non-negative dividends). Write a method: public boolean isValidISBN(long isbn) that returns true if isbn represents a valid ISBN, false otherwise. (Thanks to Katerina Soltan.)