To install a batch of parts, A works alone for (1/3) hour, and B alone works for (1/4) hour. If A and B cooperate, how many hours will it take to complete it?
安装一批零件，甲单独做用(1/3)小时，乙单独做用(1/4)小时．如果甲乙合作，要用多少小时完成．
<gadget id="calculator">1 / 3</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">1 / (1/3)</gadget>
<output>3</output>

<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">1 / (1/4)</gadget>
<output>4</output>

<gadget id="calculator">3 + 4</gadget>
<output>7</output>

<gadget id="calculator">1 / 7</gadget>
<output>1/7 = around 0.142857</output>

<result>1/7 = around 0.142857</result>