Master Li processes a batch of parts. The ratio of the number of parts completed in the first two days to the total number of parts is 1:4. If another 30 parts are processed, half of this batch of parts will be completed. How many parts are there in this batch?
李师傅加工一批零件，头二天完成的个数与零件的总个数的比是1：4，如果再加工30个，就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 / 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">30 / (1/4)</gadget>
<output>120</output>

<result>120</result>