A set of "Journey to the West" has a total of 6 copies. If you buy a set, each set costs 84 yuan. If you don't buy a complete set, each copy costs 17 yuan. How to buy cheaper? How much is it cheaper per instinct?
《西游记》一套共6本，如果买一套，每套84元，如果不成套买，每本17元．怎样买比较便宜？每本能便宜多少元？
<gadget id="calculator">17 * 6</gadget>
<output>102</output>

<gadget id="calculator">102 - 84</gadget>
<output>18</output>

<gadget id="calculator">18 / 6</gadget>
<output>3</output>

<result>3</result>