It takes 20 days for A to complete a batch of parts alone. A and B worked together to complete 90% of the batch of parts in 8 days. At this time, B made 3600 parts. How many parts are in this batch?
一批零件，甲单独完成需要20天．甲、乙两人合作8天完成这批零件的90%，这时，乙做了3600个零件．这批零件共多少个？
<gadget id="calculator">90 / 100</gadget>
<output>9/10 = around 0.9</output>

<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">(1/20) * 8</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">(9/10) - (2/5)</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">3_600 / (1/2)</gadget>
<output>7_200</output>

<result>7_200</result>