In 398-(□-81.2)=89.6, □=how much.
在398-(□-81.2)=89.6中，□=多少．
<gadget id="calculator">398 - 89.6</gadget>
<output>308.4</output>

<gadget id="calculator">308.4 + 81.2</gadget>
<output>389.6</output>

<result>389.6</result>