Master Zhang processes a batch of parts. The ratio of the number of completed parts to the number of unfinished parts on the first day is 1:4. If 15 more parts are processed, half of this batch of parts will be completed. How many parts did Master Zhang complete on the first day? Component?
张师傅加工一批零件，第一天完成的个数与未完成的个数比是1：4，如果再加工15个，就完成了这批零件的一半，张师傅第一天完成了多少个零件？
<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">1 + 4</gadget>
<output>5</output>

<gadget id="calculator">1 / 5</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">(1/2) - (1/5)</gadget>
<output>3/10 = around 0.3</output>

<gadget id="calculator">15 / (3/10)</gadget>
<output>50</output>

<gadget id="calculator">50 * (1/5)</gadget>
<output>10</output>

<result>10</result>