For a batch of parts, (1/7) of the total is completed in three days, and ((())/(())) of the total is completed every day, how many days can complete (1/3) of the total.
一批零件，三天正好完成了总数的(1/7)，每天完成总数的((())/(()))，多少天可以完成总数的(1/3)．
<gadget id="calculator">1 / 3</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">1 / 7</gadget>
<output>1/7 = around 0.142857</output>

<gadget id="calculator">(1/7) / 3</gadget>
<output>1/21 = around 0.047619</output>

<gadget id="calculator">(1/3) / (1/21)</gadget>
<output>7</output>

<result>7</result>