Two engineering teams, A and B, jointly dig a 7200m tunnel. Team A digs 280m from one side every day, and team B digs 320m from the other side every day. After both teams work for 4 days, team B will continue to complete the remaining tasks. How many days does team B need to dig to complete the task?
甲乙两个工程队合挖一条7200m的隧道．甲队从一侧每天挖280m，乙队从另一侧每天挖320m，两队都工作4天后，余下的任务由乙队接着完成，乙队还需要挖多少天才能完成任务？
<gadget id="calculator">280 + 320</gadget>
<output>600</output>

<gadget id="calculator">600 * 4</gadget>
<output>2_400</output>

<gadget id="calculator">7_200 - 2_400</gadget>
<output>4_800</output>

<gadget id="calculator">4_800 / 320</gadget>
<output>15</output>

<result>15</result>