Master Xu processes a batch of parts. The ratio of the number of parts processed on the first day to the total number of parts in this batch is 1:3. If 21 more are processed, just half of this batch of parts will be completed. How many parts are there in this batch?
徐师傅加工一批零件，第一天加工的零件个数与这批零件总数的比是1：3．如果再加工21个，正好完成这批零件的一半．这批零件共有多少个？
<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">1 + 3</gadget>
<output>4</output>

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

<gadget id="calculator">(1/2) - (1/4)</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">21 / (1/4)</gadget>
<output>84</output>

<result>84</result>