A project alone takes 12 hours and B alone takes 15 hours. After A and B work together for 1 hour, A works alone for 1 hour, and then B works alone for 1 hour, ..., A and B alternate in this way, how many hours will it take to complete the project.
一项工程，甲单独完成需12小时，乙单独完成需15小时．甲乙合做1小时后，由甲单独做1小时，再由乙单独做1小时，…，甲、乙如此交替下去，则完成该工程共用多少小时．
<gadget id="calculator">2 * 5</gadget>
<output>10</output>

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

<gadget id="calculator">1 / 12</gadget>
<output>1/12 = around 0.083333</output>

<gadget id="calculator">(1/15) + (1/12)</gadget>
<output>3/20 = around 0.15</output>

<gadget id="calculator">(3/20) * 5</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">1 - (1/15) - (1/12) - (3/4) - (1/12)</gadget>
<output>1/60 = around 0.016667</output>

<gadget id="calculator">(1/60) / (1/15)</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">1 + 10 + 1 + (1/4)</gadget>
<output>49/4 = around 12.25</output>

<result>49/4 = around 12.25</result>