The clock knocks 3 times at 3 o'clock, and it takes 6 seconds to knock it out; then it knocks 7 times at 7 o'clock, how many seconds it takes to knock it out.
时钟3点敲3下，6秒钟敲完；那么7点敲7下，多少秒钟敲完．
<gadget id="calculator">3 - 1</gadget>
<output>2</output>

<gadget id="calculator">6 / 2</gadget>
<output>3</output>

<gadget id="calculator">7 - 1</gadget>
<output>6</output>

<gadget id="calculator">3 * 6</gadget>
<output>18</output>

<result>18</result>