There are 4 different science and technology books and 5 different literature and art books on the bookshelf. Zhang Meng wants to borrow two books of different types. How many different borrowing methods are there?
书架上有4本不同的科技书和5本不同的文艺书，张萌想借两本不同类的书，共有多少种不同的借法．
<gadget id="calculator">4 * 5</gadget>
<output>20</output>

<result>20</result>