There are several pieces of the same commodity. If each piece is sold for 12 yuan, the total profit is 84 yuan. If each piece is sold for 9 yuan, the total profit is 12 yuan. What is the cost of each of these commodities?
有同样的商品若干件，如果每件卖12元，共可盈利84元，如果每件卖9元，共可盈利12元，这些商品的成本每件多少元．
<gadget id="calculator">84 - 12</gadget>
<output>72</output>

<gadget id="calculator">12 - 9</gadget>
<output>3</output>

<gadget id="calculator">72 / 3</gadget>
<output>24</output>

<gadget id="calculator">84 / 24</gadget>
<output>7/2 = around 3.5</output>

<gadget id="calculator">12 - (7/2)</gadget>
<output>17/2 = around 8.5</output>

<result>17/2 = around 8.5</result>