(3/9)+□=(8/9), how much should be filled in □
(3/9)+□=(8/9)，□里应该填多少
<gadget id="calculator">8 / 9</gadget>
<output>8/9 = around 0.888889</output>

<gadget id="calculator">3 / 9</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">(8/9) - (1/3)</gadget>
<output>5/9 = around 0.555556</output>

<result>5/9 = around 0.555556</result>