The sixth grade in the new semester will carry out the cleaning activities. If it takes 5 hours for girls to complete it alone, and 7 hours for boys to complete it alone, how long will it take for boys and girls to clean together?
新学期六年级开展大扫除活动，如果单独让女生完成需要5个小时，而单独交给男生完成需要7个小时，请问：男、女生一块儿打扫需要多长时间？
<gadget id="calculator">1 / 5</gadget>
<output>1/5 = around 0.2</output>

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

<gadget id="calculator">(1/5) + (1/7)</gadget>
<output>12/35 = around 0.342857</output>

<gadget id="calculator">1 / (12/35)</gadget>
<output>35/12 = around 2.916667</output>

<result>35/12 = around 2.916667</result>