For a project, it takes 20 days for team A to complete alone, and 30 days for team B to complete alone. Now, team A will construct one day first, and then team B will take over from team A to replace team B with one day of construction. In this way, team B will finish at last. Construction, how many days will it take to complete the task?
一项工程，甲队单独做需要20天完成，乙队单独做需要30天完成，现在先由甲队施工一天，然后由乙队接替甲队接替乙队施工一天，如此交替，最后乙队结束施工，那么共需要多少天完成任务．
<gadget id="calculator">60 / 5</gadget>
<output>12</output>

<gadget id="calculator">12 + 12</gadget>
<output>24</output>

<result>24</result>