When processing a batch of parts, the ratio of the number completed on the first day to the total number is 2:5. If 18 more parts are processed, it is exactly half completed. How many parts are there in this batch?
加工一批零件，第一天完成的个数与总个数的比是2：5，如再加工18个，就正好完成了一半，这批零件共多少个？
<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

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

<gadget id="calculator">(1/2) - (2/5)</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">18 / (1/10)</gadget>
<output>180</output>

<result>180</result>