Two engineering teams work together to complete a project, which can be completed in 12 days. After the two teams cooperate for 3 days, team B leaves due to something, and team A completes the remaining work in 15 days. Find the work efficiency of A.
两支工程队合作完成一项工程，12天可以完成，两队合作3天后，乙队因有事离开，余下的工作甲队用15天完成，求甲的工作效率．
<gadget id="calculator">1 / 12</gadget>
<output>1/12 = around 0.083333</output>

<gadget id="calculator">(1/12) * 3</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">1 - (1/4)</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">(3/4) / 15</gadget>
<output>1/20 = around 0.05</output>

<result>1/20 = around 0.05</result>