A job can be done by A in 12 hours alone, and by B in 20 hours. Now A works for 1 hour, B for 2 hours, A for 1 hour, and B for 2 hours. How many hours?
一项工作，甲单独做要12小时完成，乙单独做要20小时完成，现在甲工作1小时，乙工作2小时，甲1小时，乙工作2小时…的顺序交替工作，完成这项工作共要多少小时？
<gadget id="calculator">5 * 3</gadget>
<output>15</output>

<gadget id="calculator">15 + 1</gadget>
<output>16</output>

<result>16</result>