There is a project to be completed by A and B, because there is only one machine, and A and B can only operate it in turn. It will take 15 hours for A to complete this project independently, and 20 hours for B to complete this project independently. If A starts to work alternately with two people, and they work alternately every 1 hour, how long will it take to complete this project?
有一项工程由甲乙两人来完成，因为只有一台机器，甲乙只能轮流操作．甲独立完成此工程需15小时，乙独立完成此工程需20小时，如果由甲开始两人轮流操作，工作1小时轮换一次，完成此工程需多长时间？
<gadget id="calculator">8 * 2</gadget>
<output>16</output>

<gadget id="calculator">1 / 15</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">(1/15) + (1/20)</gadget>
<output>7/60 = around 0.116667</output>

<gadget id="calculator">(7/60) * 8</gadget>
<output>14/15 = around 0.933333</output>

<gadget id="calculator">1 - (14/15)</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">(1/15) / (1/15)</gadget>
<output>1</output>

<gadget id="calculator">16 + 1</gadget>
<output>17</output>

<result>17</result>