x plus 3 times 0.5 equals 21, find x.
x加上0.5的3倍等于21，求x．
<gadget id="calculator">0.5 * 3</gadget>
<output>1.5</output>

<gadget id="calculator">21 - 1.5</gadget>
<output>19.5</output>

<result>19.5</result>