The master and the apprentice make a batch of parts together. The apprentice made the total number (2/7), which is 21 fewer than the master. How many parts are there in this batch?
师徒两人合做一批零件，徒弟做了总数(2/7)，比师傅少做21个，这批零件有多少个？
<gadget id="calculator">2 / 7</gadget>
<output>2/7 = around 0.285714</output>

<gadget id="calculator">1 - (2/7) - (2/7)</gadget>
<output>3/7 = around 0.428571</output>

<gadget id="calculator">21 / (3/7)</gadget>
<output>49</output>

<result>49</result>