Xiao Hua and Xiao Li both read a 180-page story book. Xiao Hua reads 10 pages a day, and Xiao Li reads 15 pages a day. How many days earlier does Xiao Li finish this book than Xiao Hua?
小华和小李都看一本180页的故事书，小华每天看10页，小李每天看15页，小李比小华提前几天看完这本书？
<gadget id="calculator">180 / 10</gadget>
<output>18</output>

<gadget id="calculator">180 / 15</gadget>
<output>12</output>

<gadget id="calculator">18 - 12</gadget>
<output>6</output>

<result>6</result>