The school bought three kinds of books, of which (1/5) are literature and art books. The ratio of the number of science and technology books to the number of the other two books is 1:3. It is known that there are 20 more science and technology books than literature and art books. How many books did the school buy in total?
学校买来三种图书，其中(1/5)是文艺书，科技书的本数与其它两种书本数的比是1：3，已知科技书比文艺书多20本．学校共买来多少本书？
<gadget id="calculator">1 + 3</gadget>
<output>4</output>

<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">1 / 5</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">(1/4) - (1/5)</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">20 / (1/20)</gadget>
<output>400</output>

<result>400</result>