When processing a batch of parts, the ratio of the number of completed parts to the number of unfinished parts on the first day is 1:2. If 120 more parts are processed, half of this batch of parts can be completed. How many parts are there in this batch?
加工一批零件，第一天完成的个数与未完成的个数的比是1：2，如果再加工120个，就可以完成这批零件的一半，这批零件共有几个？
<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">1 / 3</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">(1/2) - (1/3)</gadget>
<output>1/6 = around 0.166667</output>

<gadget id="calculator">120 / (1/6)</gadget>
<output>720</output>

<result>720</result>