project A worked alone for 1 day, and cooperated with B for 2 days to complete (4/7) of the whole project. The working efficiency ratio of A and B is 2:3. How many days will it take if this project is done by B alone?
某工程甲单独做了1天，再与乙合作2天，完成了全工程的(4/7)，甲、乙工作效率比2：3．如果这项工程由乙单独做要几天完成？
<gadget id="calculator">4 / 7</gadget>
<output>4/7 = around 0.571429</output>

<gadget id="calculator">1 + 2</gadget>
<output>3</output>

<gadget id="calculator">2 / 3</gadget>
<output>2/3 = around 0.666667</output>

<gadget id="calculator">3 * (2/3)</gadget>
<output>2</output>

<gadget id="calculator">2 + 2</gadget>
<output>4</output>

<gadget id="calculator">(4/7) / 4</gadget>
<output>1/7 = around 0.142857</output>

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

<result>7</result>