Master Zhang processes a batch of parts, the ratio of the number of completed parts to the number of remaining parts on the first day is 1:2. If 15 more are processed, half of the batch of parts is completed. How many parts are there in this batch?
张师傅加工一批零件，第一天完成的个数与剩下零件个数的比是1：2．如果再加工15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 / 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">15 / (1/6)</gadget>
<output>90</output>

<result>90</result>