It took 20 seconds for the clock to strike 5 times at 5 o'clock. Then knock 8 times at 8 o'clock, how many seconds to finish knocking?
时钟5点时敲5下，用了20秒．那么8点时敲8下，几秒钟敲完？
<gadget id="calculator">8 - 1</gadget>
<output>7</output>

<gadget id="calculator">5 - 1</gadget>
<output>4</output>

<gadget id="calculator">20 / 4</gadget>
<output>5</output>

<gadget id="calculator">7 * 5</gadget>
<output>35</output>

<result>35</result>