Congcong's wall clock struck 3 times at 3 o'clock, and it took 6 seconds to finish. How many seconds does it take to knock 7 times at 7 o'clock?
聪聪家的挂钟3时敲了3下，6秒钟敲完．7时敲7下，需要多少秒钟．
<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>