When two addends are added together with their sum, the result is 1.8. If one of the addends is 0.5, what is the other addend.
两个加数与它们的和连加，得数是1.8，如果其中一个加数是0.5，则另一个加数是多少．
<gadget id="calculator">1.8 / 2</gadget>
<output>0.9</output>

<gadget id="calculator">0.9 - 0.5</gadget>
<output>0.4</output>

<result>0.4</result>