Uncle Wang wants to process a batch of parts. He processed 50 parts in the morning, and the pass rate is 80%. How many parts must he process to make the pass rate of Uncle Wang's parts reach 90%?
王叔叔要加工一批零件，上午加工了50个，合格率是80%，要想使王叔叔加工零件的合格率达90%，还要加工多少个？
<gadget id="calculator">80 / 100</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">1 - (4/5)</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">90 / 100</gadget>
<output>9/10 = around 0.9</output>

<gadget id="calculator">1 - (9/10)</gadget>
<output>1/10 = around 0.1</output>

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

<gadget id="calculator">50 * 2</gadget>
<output>100</output>

<gadget id="calculator">100 - 50</gadget>
<output>50</output>

<result>50</result>