A project A and B cooperate to complete (7/10) of the whole project, and the rest is completed by A alone. A has done 21 days in total. This project needs 30 days for A to do alone. If B does it alone, How many days?
一项工程甲、乙合作完成了全工程的(7/10)，剩下的由甲单独完成，甲一共做了21天，这项工程由甲单独做需30天，如果由乙单独做，需多少天？
<gadget id="calculator">7 / 10</gadget>
<output>7/10 = around 0.7</output>

<gadget id="calculator">1 / 30</gadget>
<output>1/30 = around 0.033333</output>

<gadget id="calculator">1 - (7/10)</gadget>
<output>3/10 = around 0.3</output>

<gadget id="calculator">(3/10) / (1/30)</gadget>
<output>9</output>

<gadget id="calculator">21 - 9</gadget>
<output>12</output>

<gadget id="calculator">(1/30) * 12</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">(7/10) - (2/5)</gadget>
<output>3/10 = around 0.3</output>

<gadget id="calculator">(3/10) / 12</gadget>
<output>1/40 = around 0.025</output>

<gadget id="calculator">1 / (1/40)</gadget>
<output>40</output>

<result>40</result>