When x=how much, 7: ((1/5)x-4) is meaningless.
当x=多少时，7：((1/5)x-4)没有意义．
<gadget id="calculator">4 + 0</gadget>
<output>4</output>

<gadget id="calculator">1 / 5</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">4 / (1/5)</gadget>
<output>20</output>

<result>20</result>