My mother sews buttons for a batch of jackets. If she sews 15 buttons a day, the work can be completed on schedule. If she sews 18 buttons a day, the work can be completed 3 days ahead of schedule. How many coats are there in this batch?
妈妈给一批上衣缝扣子，如果每天缝15件，正好可以按期完工，如果每天缝18件，就可提前3天完工．这批上衣共有多少件？
<gadget id="calculator">1 / 15</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">1 / 18</gadget>
<output>1/18 = around 0.055556</output>

<gadget id="calculator">(1/15) - (1/18)</gadget>
<output>1/90 = around 0.011111</output>

<gadget id="calculator">3 / (1/90)</gadget>
<output>270</output>

<result>270</result>