10 years, because In 5 years after 2008, 2013 you get new math books 5 more years you get another set of math books-2018, but 2008 plus 10 years you get science books which is also 2018 which is also when you get another set of math books. So the next year you get math books and science books shipped the same year is 2018. 2018-2008=10 so 10 years pass
