A project is completed in 20 days jointly by A and B. The ratio of the work efficiency of A and B is 4:5. How many days will it take for A to do this project alone?
一项工程，甲、乙合做20天完成，甲、乙两队的工作效率的比是4：5．甲独做这项工程需要多少天．
<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">4 + 5</gadget>
<output>9</output>

<gadget id="calculator">4 / 9</gadget>
<output>4/9 = around 0.444444</output>

<gadget id="calculator">(1/20) * (4/9)</gadget>
<output>1/45 = around 0.022222</output>

<gadget id="calculator">1 / (1/45)</gadget>
<output>45</output>

<result>45</result>