The library has two bookshelves, A and B. There are 260 books on shelf A. When shelf B takes out 20 books and puts them on shelf A, there are 10 more books on shelf A than on shelf B. How many original books are there on the B bookshelf?
图书馆有甲、乙两个书架，甲书架有书260本，当乙书架取出20本放到甲书架后，甲书架的书比乙书架的2倍还多10本．乙书架原有书多少本．
<gadget id="calculator">20 - 10</gadget>
<output>10</output>

<gadget id="calculator">260 + 10</gadget>
<output>270</output>

<gadget id="calculator">270 / 2</gadget>
<output>135</output>

<result>135</result>