The shoe store buys several pairs of leather shoes from the wholesale department at a unit price of 25 yuan. Sell at the retail price of 40 yuan a pair, and when you sell half and 15 pairs, the cost has been recovered. Ask how many pairs of leather shoes to buy?
鞋店从批发部以25元的单价购进皮鞋若干双．按40元一双的零售价卖出，卖出一半又15双时，已将成本收回．问购进皮鞋多少双？
<gadget id="calculator">40 * 15</gadget>
<output>600</output>

<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">40 * (1/2)</gadget>
<output>20</output>

<gadget id="calculator">25 - 20</gadget>
<output>5</output>

<gadget id="calculator">600 / 5</gadget>
<output>120</output>

<result>120</result>