It takes 40 days for Team A to complete a project alone. If team B works for 10 days first, and the rest of the project will be completed by the cooperation of teams A and B, it will take another 20 days. How many days will it take if Team B completes the project alone?
一项工程，甲队单独完成需40天．若乙队先做10天，余下的工程由甲、乙两队合作，又需20天可完成．如果乙队单独完成此工程，则需多少天．
<result>60</result>