My mother sews buttons for a batch of jackets. If she sews 15 buttons a day, she will finish 2 days later than the stipulated time limit; if she sews 18 buttons a day, she will finish 3 days earlier than the stipulated time limit. How many coats are in this batch?
妈妈给一批上衣缝纽扣，如果每天缝15件，就比规定的工期晚2天完成；如果每天缝18件，就可比规定的工期提前3天完成．这批上衣共多少件？
<gadget id="calculator">2 + 3</gadget>
<output>5</output>

<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">5 / (1/90)</gadget>
<output>450</output>

<result>450</result>