When a=3, b=1.5, 3a-3b=how much.
当a=3，b=1.5时，3a-3b=多少．
<gadget id="calculator">3 * 3</gadget>
<output>9</output>

<gadget id="calculator">3 * 1.5</gadget>
<output>4.5</output>

<gadget id="calculator">9 - 4.5</gadget>
<output>4.5</output>

<result>4.5</result>