It is known that 6 is the median item in the ratio of a number to 9, then what is the number.
已知6是一个数和9的比例中项，则这个数是多少．
<gadget id="calculator">6 * 6</gadget>
<output>36</output>

<gadget id="calculator">36 / 9</gadget>
<output>4</output>

<result>4</result>