If 5x+4=8.5, then 4-2x=how much
如果5x+4=8.5，则4-2x=多少
<gadget id="calculator">8.5 - 4</gadget>
<output>4.5</output>

<gadget id="calculator">4.5 / 5</gadget>
<output>0.9</output>

<gadget id="calculator">2 * 0.9</gadget>
<output>1.8</output>

<gadget id="calculator">4 - 1.8</gadget>
<output>2.2</output>

<result>2.2</result>