There are 777 books on a seven-story bookshelf, and each floor is 7 books less than the one below it. Ask: How many books are on the top floor?
一个七层书架放了777本书，每一层比它的下一层少7本书，问：最上面一层放了多少本书？
<gadget id="calculator">777 / 7</gadget>
<output>111</output>

<gadget id="calculator">7 - 4</gadget>
<output>3</output>

<gadget id="calculator">7 * 3</gadget>
<output>21</output>

<gadget id="calculator">111 - 21</gadget>
<output>90</output>

<result>90</result>