Add 9 to a number, multiply by 9, subtract 9, and finally divide by 9, the result is still 9. What is this number?
一个数加9，乘9，减9，最后除以9，结果还是9．这个数是多少？
<gadget id="calculator">9 * 9</gadget>
<output>81</output>

<gadget id="calculator">81 + 9</gadget>
<output>90</output>

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

<gadget id="calculator">10 - 9</gadget>
<output>1</output>

<result>1</result>