Knock 3 times at 3 o'clock and finish knocking in 6 seconds. How many seconds does it take to knock 8 times at 8 o'clock?
3时敲3下，6秒钟敲完．8时敲8下，需要多少秒钟敲完．
<gadget id="calculator">3 - 1</gadget>
<output>2</output>

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

<gadget id="calculator">8 - 1</gadget>
<output>7</output>

<gadget id="calculator">3 * 7</gadget>
<output>21</output>

<result>21</result>