If 4x+9=19.4, then 4x-9=how much.
如果4x+9=19.4，那么4x-9=多少．
<gadget id="calculator">19.4 - 9</gadget>
<output>10.4</output>

<gadget id="calculator">10.4 - 9</gadget>
<output>1.4</output>

<result>1.4</result>