Known: (81/(5-□*(1/3)))*(5/9)=15, then □=how much.
已知：(81/(5-□*(1/3)))*(5/9)=15，则□=多少．
<gadget id="calculator">15 * 5</gadget>
<output>75</output>

<gadget id="calculator">5 / 9</gadget>
<output>5/9 = around 0.555556</output>

<gadget id="calculator">81 * (5/9)</gadget>
<output>45</output>

<gadget id="calculator">75 - 45</gadget>
<output>30</output>

<gadget id="calculator">1 / 3</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">15 * (1/3)</gadget>
<output>5</output>

<gadget id="calculator">30 / 5</gadget>
<output>6</output>

<result>6</result>