It takes 6 seconds for the clock to strike 3 times at 3 o'clock, how many seconds does it take to strike 7 times at 7 o'clock.
时钟3时敲3下，用时6秒，7时敲7下用时多少秒．
<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>