Dad goes to two bookstores to buy a book with a price tag of 90 yuan. Bookstore A will sell it with a 30% discount, and bookstore B will get a discount of 20 yuan if it is over 80 yuan. If Dad buys this book, which bookstore can I buy it cheaply? How much cheaper?
爸爸到两家书店买一本标价都是90元的书，A书店打七折销售，B书店满80元减20元．如果爸爸买这本书，在哪个书店买便宜？便宜多少钱？
<gadget id="calculator">90 - 20</gadget>
<output>70</output>

<gadget id="calculator">70 / 100</gadget>
<output>7/10 = around 0.7</output>

<gadget id="calculator">90 * (7/10)</gadget>
<output>63</output>

<gadget id="calculator">70 - 63</gadget>
<output>7</output>

<result>7</result>