Both master and apprentice assemble computers at the same time, the master assembles 12 computers every day, and the apprentice assembles 5 computers every day. As a result, after a few days, the master assembled 21 more computers than the apprentice.
师徒两人同时装配计算机，师傅每天装配12台，徒弟每天装配5台．结果多少天后师傅比徒弟多装配21台计算机．
<gadget id="calculator">12 - 5</gadget>
<output>7</output>

<gadget id="calculator">21 / 7</gadget>
<output>3</output>

<result>3</result>