When processing a batch of parts, the ratio of completed parts to the total number of parts is 2:5. If 50 more parts are processed, the ratio of completed parts to the total number of parts is 3:5. How many parts are there in this batch?
加工一批零件，已完成个数与零件总个数的比是2：5，如果再加工50个，那么完成个数与零件总个数的比是3：5．这批零件共有多少个？
<gadget id="calculator">3 / 5</gadget>
<output>3/5 = around 0.6</output>

<gadget id="calculator">2 / 5</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">(3/5) - (2/5)</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">50 / (1/5)</gadget>
<output>250</output>

<result>250</result>