For a commodity, 13 pieces are sold on the first day, with a profit of 7 yuan per piece; 12 pieces are sold on the second day, with a profit of 11 yuan per piece. If the total amount of sales in these two days is the same, then the purchase price of this commodity is how many yuan per piece.
一种商品，第一天卖出13件，每件利润7元；第二天卖出12件，每件利润11元．如果这两天的售货总金额是一样多，那么这种商品的进货价格是每件多少元．
<gadget id="calculator">11 * 12</gadget>
<output>132</output>

<gadget id="calculator">7 * 13</gadget>
<output>91</output>

<gadget id="calculator">132 - 91</gadget>
<output>41</output>

<gadget id="calculator">13 - 12</gadget>
<output>1</output>

<gadget id="calculator">41 / 1</gadget>
<output>41</output>

<result>41</result>