job that A alone takes 30 days to complete, B's work efficiency is twice that of C. Due to mutual interference, when any two work together, their respective work efficiency will decrease by 25%. Now A and B cooperate for 7 days first, and A and C cooperate for 13 days to complete the task. How many days will it take to complete this work if B and C cooperate?
一项工作，甲单独做用30天完成，乙的工作效率是丙的2倍．由于相互的干扰，任意两个共同工作时，各自的工作效率均降低25%，现由甲、乙先合作7天，甲、丙再合作13天后完成任务．如果这项工作由乙、丙合作要多少天完成？
<gadget id="calculator">1 + 2</gadget>
<output>3</output>

<gadget id="calculator">6 / 3</gadget>
<output>2</output>

<gadget id="calculator">2 + 7</gadget>
<output>9</output>

<gadget id="calculator">9 * 2</gadget>
<output>18</output>

<result>18</result>