5*□/3/4-291=2009, □=how much.
5*□/3/4-291=2009，□=多少．
<gadget id="calculator">2_009 + 291</gadget>
<output>2_300</output>

<gadget id="calculator">12 - 291</gadget>
<output>-279</output>

<gadget id="calculator">(-279) + 291</gadget>
<output>12</output>

<gadget id="calculator">2_300 * 12</gadget>
<output>27_600</output>

<gadget id="calculator">27_600 / 5</gadget>
<output>5_520</output>

<result>5_520</result>