It takes 21 minutes to saw 8 sections of a piece of wood. How many minutes will it take to saw 13 sections? (Solution with proportional knowledge)
一根木料，锯8段需要21分钟，如果锯13段，需要多少分钟？(用比例知识解)
<gadget id="calculator">21 * 13</gadget>
<output>273</output>

<gadget id="calculator">273 - 21</gadget>
<output>252</output>

<gadget id="calculator">8 - 1</gadget>
<output>7</output>

<gadget id="calculator">252 / 7</gadget>
<output>36</output>

<result>36</result>