A worker originally planned to produce 9 machine parts per day, which would take 20 days to complete. If it is completed in 15 days, how many more should be produced per day?
某工人原计划每天生产机器零件9个，20天完成．如果15天完成，每天应该多生产多少个？
<gadget id="calculator">9 * 20</gadget>
<output>180</output>

<gadget id="calculator">9 * 15</gadget>
<output>135</output>

<gadget id="calculator">180 - 135</gadget>
<output>45</output>

<gadget id="calculator">45 / 15</gadget>
<output>3</output>

<result>3</result>