When a theater troupe goes to perform in other places, it needs to pay 60 yuan for the theater rent for one day off. After deducting the rent and miscellaneous expenses for one day of performance, the average income can be 240 yuan. Now I rent the theater for 30 days, and the total performance income is 4200 yuan. How many days does this troupe perform?
一个剧团去外地演出，休息一天，就要付出60元的剧场租金，演出一天，扣去场租、杂项开支，平均可收入240元．现租用剧场30天，演出共收入4200元，这个剧团演出多少天？
<gadget id="calculator">3_000 / 300</gadget>
<output>10</output>

<gadget id="calculator">30 - 10</gadget>
<output>20</output>

<result>20</result>