It takes 3 minutes to saw a log 4 meters long into 2 pieces, how many minutes will it take to saw it into 8 pieces?
一根长4米的圆木锯成2段要用3分钟，锯成8段要用多少分钟．
<gadget id="calculator">8 - 1</gadget>
<output>7</output>

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

<result>21</result>