The central primary school has newly purchased a batch of books and distributes them to grades four, five, and six according to the ratio of 4:5:6. It is known that grades four get 20 books less than grades six, and how many books are grades five given?
中心小学新购进一批图书，按4：5：6分给四、五、六三个年级，已知四年级比六年级少分得20本，五年级分得图书多少本？
<gadget id="calculator">6 - 4</gadget>
<output>2</output>

<gadget id="calculator">20 / 2</gadget>
<output>10</output>

<gadget id="calculator">10 * 5</gadget>
<output>50</output>

<result>50</result>