A piece of wood is 20 meters long, and it should be divided into 4 equal parts. It takes 7 minutes to saw the next section, how many minutes will it take to complete the saw.
一根木头长20米，要把它平均分成4段．每锯下一段需7分钟，锯完一共要用多少分钟．
<gadget id="calculator">4 - 1</gadget>
<output>3</output>

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

<result>21</result>