A and B each have a number of books. After A gives (1/5) to B, B then takes out (1/10) of the existing books to A. At this time, they both have 90 books. How many books does A have?
甲、乙两人各有图书若干，甲拿出(1/5)给乙后，乙再拿出现有图书的(1/10)给甲，这时他们都有图书90本．甲原有图书多少本．
<gadget id="calculator">90 + 90</gadget>
<output>180</output>

<gadget id="calculator">1 / 10</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">1 - (1/10)</gadget>
<output>9/10 = around 0.9</output>

<gadget id="calculator">90 / (9/10)</gadget>
<output>100</output>

<gadget id="calculator">180 - 100</gadget>
<output>80</output>

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

<gadget id="calculator">1 - (1/5)</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">80 / (4/5)</gadget>
<output>100</output>

<result>100</result>