The retail price of "Andersen's Fairy Tales" is 17 yuan each, and it costs 90 yuan to buy 6 copies of the same book online. Which is cheaper, online shopping or retail? How much is it cheaper?
《安徒生童话》零售价每本17元，买同样的书网购6本需要90元，网购和零售哪种便宜些？便宜多少元？
<gadget id="calculator">90 / 6</gadget>
<output>15</output>

<gadget id="calculator">17 - 15</gadget>
<output>2</output>

<result>2</result>