The ratio of the original number of books on the two bookshelves A and B is 2:3, if you take 20 books from the B bookshelf and put them on the A bookshelf, the number of books on the two bookshelves will be the same. How many books are there on shelf A?
甲、乙两个书架原有图书本数的比是2：3，从乙书架拿20本放到甲书架，两个书架上图书本数就一样多．原来甲书架有多少本图书？
<gadget id="calculator">20 + 20</gadget>
<output>40</output>

<gadget id="calculator">3 / 2</gadget>
<output>3/2 = around 1.5</output>

<gadget id="calculator">(3/2) - 1</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">40 / (1/2)</gadget>
<output>80</output>

<result>80</result>