A batch of saplings, A and B are planted in 15 days, B and C are planted in 12 days, A and C are planted in 8 days. If A, B, and C are planted one day each in turn in the order of A, B, and C, how many days will it take to plant these saplings?
一批树苗，甲乙两人15天栽完，乙丙两人12天栽完，甲丙两人8天栽完．若按甲乙丙的顺序轮流各栽一天，栽完这批树苗共需多少天？
<gadget id="calculator">7 * 3</gadget>
<output>21</output>

<gadget id="calculator">21 + 1</gadget>
<output>22</output>

<result>22</result>