Xiao Ming and Xiao Ling copied a manuscript, Xiao Ming first copied it for 12 hours, and then they both copied it together, and it took 9 hours to complete it; if Xiao Ling copied it first for 12 hours, and then they copied it together, it took another 7 hours to complete it. Now, how many hours will it take for two people to copy this manuscript at the same time?
小明和小玲抄一份稿件，先由小明抄12小时，然后两人合抄，9小时可以完成；如果先由小玲抄12小时，然后两人合抄，还要7小时可以完成．现在，由两人同时合抄这份稿件，需要几小时才能完成？
<gadget id="calculator">12 + 9</gadget>
<output>21</output>

<gadget id="calculator">21 + 7</gadget>
<output>28</output>

<gadget id="calculator">28 / 2</gadget>
<output>14</output>

<result>14</result>